Fix compiler warning type-limits in found_string (ref #2911) (#3263)

Whether `char` has a sign is implementation dependent, and `c >= 0` may
result in a compiler warning. We can use bitwise operator to check
whether the character is a control character, regardless of the sign.

src/jv_parse.c

@@ -493,7 +493,7 @@ static pfunc found_string(struct jv_parser* p) {
         return "Invalid escape";
       }
     } else {
-      if (c >= 0 && c <= 0x001f)
+      if (!(c & ~0x1F))
         return "Invalid string: control characters from U+0000 through U+001F must be escaped";
       *out++ = c;
     }