cgen: fix premature variable release by autofree (vlang#20731)

vlib/v/gen/c/cgen.v

@@ -2970,7 +2970,9 @@ fn (mut g Gen) autofree_scope_vars2(scope &ast.Scope, start_pos int, end_pos int
 				// continue
 				// }
 				// if v.pos.pos > end_pos {
-				if obj.pos.pos > end_pos || (obj.pos.pos < start_pos && obj.pos.line_nr == line_nr) {
+				if obj.pos.pos > end_pos
+					|| (obj.pos.pos < start_pos && obj.pos.line_nr == line_nr)
+					|| (end_pos < scope.end_pos && obj.expr is ast.IfExpr) {
 					// Do not free vars that were declared after this scope
 					continue
 				}

vlib/v/slow_tests/valgrind/if_expr_autofree_optional.v

@@ -0,0 +1,19 @@
+fn main() {
+	out := if temp := f('') {
+		temp
+	} else if temp := f('something') {
+		temp
+	} else {
+		return
+	}
+
+	assert out[0] == 104
+	assert out[1] == 105
+}
+
+fn f(s string) ?[]u8 {
+	if s == '' {
+		return none
+	}
+	return [u8(104), 105]
+}