Fix CFRelease on NULL if CGImageSourceRef create failed

Fix CFRelease on NULL if CGImageSourceRef create failed, this may happen on progressive download with wrong image data

@@ -376,8 +376,10 @@ didReceiveResponse:(NSURLResponse *)response
             }
         }
 
+        if (imageSource) {
             CFRelease(imageSource);
         }
+    }
 
     for (SDWebImageDownloaderProgressBlock progressBlock in [self callbacksForKey:kProgressCallbackKey]) {
         progressBlock(self.imageData.length, self.expectedSize, self.request.URL);