Show invalid table tags as hex in error messages.

This avoids the risk of the returned message including NUL or other
non-printing characters.

(See https://bugzilla.mozilla.org/show_bug.cgi?id=1937227 for the
example that motivated this change.)

src/ots.cc

@@ -102,11 +102,18 @@ bool CheckTag(uint32_t tag_value) {
 
 namespace {
 
-#define OTS_MSG_TAG_(level,otf_,msg_,tag_) \
-  (OTS_MESSAGE_(level,otf_,"%c%c%c%c: %s", OTS_UNTAG(tag_), msg_), false)
+// ots_msg_tag emits the tag as hex if it is not valid, to avoid putting NUL or
+// other non-printables into the message string.
+static inline void ots_msg_tag(int level, const ots::FontFile* otf, const char* msg, uint32_t tag) {
+  if (ots::CheckTag(tag)) {
+    OTS_MESSAGE_(level, otf, "%c%c%c%c: %s", OTS_UNTAG(tag), msg);
+  } else {
+    OTS_MESSAGE_(level, otf, "<%08X>: %s", tag, msg);
+  }
+}
 
 // Generate a message with or without a table tag, when 'header' is the FontFile pointer
-#define OTS_FAILURE_MSG_TAG(msg_,tag_) OTS_MSG_TAG_(0, header, msg_, tag_)
+#define OTS_FAILURE_MSG_TAG(msg_,tag_) (ots_msg_tag(0, header, msg_, tag_), false)
 #define OTS_FAILURE_MSG_HDR(...)       OTS_FAILURE_MSG_(header, __VA_ARGS__)
 #define OTS_WARNING_MSG_HDR(...)       OTS_WARNING_MSG_(header, __VA_ARGS__)