Protect against malformed image

Don’t add nil to the array, just skip the frame.

SDWebImage/UIImage+GIF.m

@@ -32,6 +32,8 @@
 
         for (size_t i = 0; i < count; i++) {
             CGImageRef image = CGImageSourceCreateImageAtIndex(source, i, NULL);
+            if (!image)
+                continue;
 
             duration += [self sd_frameDurationAtIndex:i source:source];