Merge pull request #2691 from zhongwuzw/fix_apng_property_crash

Fixes crash for apng coder when properties are nil

SDWebImage/SDImageAPNGCoder.m

@@ -153,6 +153,9 @@ const CFStringRef kCGImagePropertyAPNGUnclampedDelayTime = (__bridge CFStringRef
 - (float)sd_frameDurationAtIndex:(NSUInteger)index source:(CGImageSourceRef)source {
     float frameDuration = 0.1f;
     CFDictionaryRef cfFrameProperties = CGImageSourceCopyPropertiesAtIndex(source, index, nil);
+    if (!cfFrameProperties) {
+        return frameDuration;
+    }
     NSDictionary *frameProperties = (__bridge NSDictionary *)cfFrameProperties;
     NSDictionary *pngProperties = frameProperties[(NSString *)kCGImagePropertyPNGDictionary];