Merge pull request #1 from langyanduan/master

fix encoding static single PNG image

Author: dreampiggy

SDWebImageAPNGCoder/Classes/SDWebImageAPNGCoder.m

@@ -142,7 +142,7 @@ - (nullable NSData *)encodedDataWithImage:(nullable UIImage *)image format:(SDIm
     CFStringRef imageUTType = [NSData sd_UTTypeFromSDImageFormat:SDImageFormatPNG];
     NSArray<SDWebImageFrame *> *frames = [SDWebImageCoderHelper framesFromAnimatedImage:image];
 
-    CGImageDestinationRef imageDestination = CGImageDestinationCreateWithData((__bridge CFMutableDataRef)imageData, imageUTType, frames.count, NULL);
+    CGImageDestinationRef imageDestination = CGImageDestinationCreateWithData((__bridge CFMutableDataRef)imageData, imageUTType, frames.count == 0 ? 1 : frames.count, NULL);
     if (!imageDestination) {
         // Handle failure.
         return nil;