Try to ensure that width * y computations are unsigned.

UB!!! Found while looking at #179.

src/gifunopt.c

@@ -23,16 +23,16 @@ static void
 put_image_in_screen(Gif_Stream *gfs, Gif_Image *gfi, uint16_t *screen)
 {
   int transparent = gfi->transparent;
-  int x, y;
-  int w = gfi->width;
-  int h = gfi->height;
+  unsigned x, y;
+  unsigned w = gfi->width;
+  unsigned h = gfi->height;
   if (gfi->left + w > gfs->screen_width)
       w = gfs->screen_width - gfi->left;
   if (gfi->top + h > gfs->screen_height)
       h = gfs->screen_height - gfi->top;
 
   for (y = 0; y < h; y++) {
-    uint16_t *move = screen + gfs->screen_width * (y + gfi->top) + gfi->left;
+    uint16_t *move = screen + (unsigned) gfs->screen_width * (y + gfi->top) + gfi->left;
     uint8_t *line = gfi->img[y];
     for (x = 0; x < w; x++, move++, line++)
       if (*line != transparent)
@@ -45,9 +45,9 @@ static void
 put_background_in_screen(Gif_Stream *gfs, Gif_Image *gfi, uint16_t *screen)
 {
   uint16_t solid;
-  int x, y;
-  int w = gfi->width;
-  int h = gfi->height;
+  unsigned x, y;
+  unsigned w = gfi->width;
+  unsigned h = gfi->height;
   if (gfi->left + w > gfs->screen_width) w = gfs->screen_width - gfi->left;
   if (gfi->top + h > gfs->screen_height) h = gfs->screen_height - gfi->top;
 
@@ -58,7 +58,7 @@ put_background_in_screen(Gif_Stream *gfs, Gif_Image *gfi, uint16_t *screen)
       solid = TRANSPARENT;
 
   for (y = 0; y < h; y++) {
-    uint16_t *move = screen + gfs->screen_width * (y + gfi->top) + gfi->left;
+    uint16_t *move = screen + (unsigned) gfs->screen_width * (y + gfi->top) + gfi->left;
     for (x = 0; x < w; x++, move++)
       *move = solid;
   }
@@ -71,7 +71,7 @@ create_image_data(Gif_Stream *gfs, Gif_Image *gfi, uint16_t *screen,
 {
   int have[257];
   int transparent = -1;
-  unsigned pos, size = gfs->screen_width * gfs->screen_height;
+  unsigned pos, size = (unsigned) gfs->screen_width * (unsigned) gfs->screen_height;
   uint16_t *move;
   int i;
 
@@ -116,7 +116,7 @@ create_image_data(Gif_Stream *gfs, Gif_Image *gfi, uint16_t *screen,
 static int
 unoptimize_image(Gif_Stream *gfs, Gif_Image *gfi, uint16_t *screen)
 {
-  unsigned size = gfs->screen_width * gfs->screen_height;
+  unsigned size = (unsigned) gfs->screen_width * (unsigned) gfs->screen_height;
   int used_transparent;
   uint8_t *new_data = Gif_NewArray(uint8_t, size);
   uint16_t *new_screen = screen;
@@ -190,7 +190,7 @@ Gif_FullUnoptimize(Gif_Stream *gfs, int flags)
     return 0;
 
   Gif_CalculateScreenSize(gfs, 0);
-  size = gfs->screen_width * gfs->screen_height;
+  size = (unsigned) gfs->screen_width * (unsigned) gfs->screen_height;
 
   screen = Gif_NewArray(uint16_t, size);
   gfi = gfs->images[0];

src/gifwrite.c

@@ -420,7 +420,7 @@ write_compressed_data(Gif_Stream *gfs, Gif_Image *gfi,
   unsigned pos;
   unsigned clear_bufpos, clear_pos;
   unsigned line_endpos;
-  const unsigned image_endpos = gfi->height * gfi->width;
+  const unsigned image_endpos = (size_t) gfi->height * (size_t) gfi->width;
   const uint8_t *imageline;
 
   unsigned run = 0;

src/ungifwrt.c

@@ -329,7 +329,7 @@ write_compressed_data(Gif_Image *gfi,
 
 #ifndef GIF_NO_COMPRESSION
         if (!do_clear) {
-          unsigned pixels_left = gfi->width * gfi->height - pos;
+          unsigned pixels_left = (unsigned) gfi->width * (unsigned) gfi->height - pos;
           if (pixels_left) {
             /* Always clear if run_ewma gets small relative to
                min_code_bits. Otherwise, clear if #images/run is smaller

src/xform.c

@@ -324,7 +324,7 @@ rotate_image(Gif_Image* gfi, Gt_Frame* fr, int rotation)
   int width = gfi->width;
   int height = gfi->height;
   uint8_t **img = gfi->img;
-  uint8_t *new_data = Gif_NewArray(uint8_t, (unsigned) width * height);
+  uint8_t *new_data = Gif_NewArray(uint8_t, (unsigned) width * (unsigned) height);
   uint8_t *trav = new_data;
 
   /* this function can only rotate by 90 or 270 degrees */
@@ -390,7 +390,7 @@ static void kcscreen_init(kcscreen* kcs, Gif_Stream* gfs, int sw, int sh) {
     assert(!kcs->data && !kcs->scratch);
     kcs->width = sw <= 0 ? gfs->screen_width : sw;
     kcs->height = sh <= 0 ? gfs->screen_height : sh;
-    sz = (unsigned) kcs->width * kcs->height;
+    sz = kcs->width * kcs->height;
     kcs->data = Gif_NewArray(kacolor, sz);
     if ((gfs->nimages == 0 || gfs->images[0]->transparent < 0)
         && gfs->global && gfs->background < gfs->global->ncol) {
@@ -476,7 +476,7 @@ static void ksscreen_init(ksscreen* kss, Gif_Stream* gfs, int sw, int sh) {
     assert(!kss->data && !kss->scratch);
     kss->width = sw <= 0 ? gfs->screen_width : sw;
     kss->height = sh <= 0 ? gfs->screen_height : sh;
-    sz = (unsigned) kss->width * kss->height;
+    sz = kss->width * kss->height;
     kss->data = Gif_NewArray(scale_color, sz);
     if ((gfs->nimages == 0 || gfs->images[0]->transparent < 0)
         && gfs->global && gfs->background < gfs->global->ncol) {
@@ -1084,7 +1084,7 @@ static void scale_image_data_weighted(scale_context* sctx, Gif_Image* gfo,
         /* skip */;
     for (yi = yi0; yi != yi1; ++yi) {
         const scale_color* iscr = &sctx->iscr.data[sctx->iscr.width * yi];
-        scale_color* oscr = &kcx[gfo->width * yi];
+        scale_color* oscr = &kcx[(unsigned) gfo->width * yi];
         for (xo = 0; xo != gfo->width; ++xo)
             sc_clear(&oscr[xo]);
         for (w = ww; w->opos < gfo->left + gfo->width; ++w)
@@ -1098,7 +1098,7 @@ static void scale_image_data_weighted(scale_context* sctx, Gif_Image* gfo,
         for (xo = 0; xo != gfo->width; ++xo)
             sc_clear(&sc[xo]);
         for (; w->opos < gfo->top + yo + 1; ++w) {
-            const scale_color* iscr = &kcx[gfo->width * w->ipos];
+            const scale_color* iscr = &kcx[(unsigned) gfo->width * w->ipos];
             assert(w->ipos >= yi0 && w->ipos < yi1);
             for (xo = 0; xo != gfo->width; ++xo)
                 SCVEC_ADDVxF(sc[xo], iscr[xo], w->w);