Fix build break introduced from libpng17

12->12U in a compare when the other side is signed.

Signed-off-by: John Bowler <jbowler@acm.org>

pngrutil.c

@@ -1948,7 +1948,7 @@ png_handle_pCAL(png_structrp png_ptr, png_inforp info_ptr)
    /* We need to have at least 12 bytes after the purpose string
     * in order to get the parameter information.
     */
-   if (endptr - buf <= 12U)
+   if (endptr - buf <= 12)
    {
       png_chunk_benign_error(png_ptr, "invalid");
       return;