Fix the limitBytes options does not work as expected because of type error

[JiaqiSun]

NSUInteger limitBytes = 0;
NSNumber *limitBytesValue = options[SDImageCoderDecodeScaleDownLimitBytes];
if (limitBytesValue != nil) {
limitBytes = limitBytesValue.unsignedIntegerValue;
}
_limitBytes = limitBytes; ❌❌❌ **ERROR: limitBytes = 62914560, but _limitBytes = 1。**❌❌❌
// Scale down to limit bytes if need
if (_limitBytes > 0) {
// Hack 32 BitsPerPixel
CGSize imageSize = CGSizeMake(_canvasWidth, _canvasHeight);
CGSize framePixelSize = SDCalculateScaleDownPixelSize(_limitBytes, imageSize, _frameCount, 4);
// Override thumbnail size
_thumbnailSize = framePixelSize;
_preserveAspectRatio = YES;
}