Sync with upstream

Even though libgd/libgd#492 is not a relevant bug fix for PHP, since
the binding doesn't use the `gdImage*Ptr()` functions at all, we're
porting the fix to stay in sync here.
void gdImageGif (gdImagePtr im, FILE * outFile)
{
  gdIOCtx *out = gdNewFileCtx (outFile);
  gdImageGifCtx (im, out);
  out->gd_free (out);
 }

void gdImageGif (gdImagePtr im, FILE * outFile)
{
  gdIOCtx *out = gdNewFileCtx (outFile);
  gdImageGifCtx (im, out);
  out->gd_free (out);
 }
