fix the crash caused by nil partialLoadHandler

Summary:
## Problem
the partialLoadHandler is nil on line 338 of RCTImageLoader, therefore, if there is a cached image, it would crash on line 495.

## Change
Check if partialLoadHandler is nil on line 495 to prevent the crash

Changelog: [iOS][Changed] - fix the crash caused by nil partialLoadHandler

Reviewed By: appden

Differential Revision: D34544090

fbshipit-source-id: f9965700e529c5add1e25867a3772c053447d99a

Author: Mo Wang

Libraries/Image/RCTImageLoader.mm

@@ -492,7 +492,7 @@ - (RCTImageURLLoaderRequest *)_loadImageOrDataWithURLRequest:(NSURLRequest *)req
   BOOL cacheResult = [loadHandler respondsToSelector:@selector(shouldCacheLoadedImages)] ?
   [loadHandler shouldCacheLoadedImages] : YES;
 
-  if (cacheResult) {
+  if (cacheResult && partialLoadHandler) {
     UIImage *image = [[self imageCache] imageForUrl:request.URL.absoluteString
                                                size:size
                                               scale:scale