Defend against nil pointer in RCTImageLoader.m

Summary:
Defend against nil pointer in RCTImageLoader.m. In some rare case, the "response" might be nil, and this would lead to crash.

This issue is related: #6952
Closes #8141

Differential Revision: D3579427

Pulled By: javache

fbshipit-source-id: 7b71768a0c6d74a42d280b1d54adda86b493f0ef

Author: yeshuang

Libraries/Image/RCTImageLoader.m

@@ -382,6 +382,9 @@ - (RCTImageLoaderCancellationBlock)_loadURLRequest:(NSURLRequest *)request
     if (error) {
       completionHandler(error, nil);
       return;
+    } else if (!response) {
+      completionHandler(RCTErrorWithMessage(@"Response metadata error"), nil);
+      return;
     } else if (!data) {
       completionHandler(RCTErrorWithMessage(@"Unknown image download error"), nil);
       return;
@@ -435,8 +438,16 @@ - (RCTImageLoaderCancellationBlock)_loadURLRequest:(NSURLRequest *)request
   // Download image
   __weak __typeof(self) weakSelf = self;
   RCTNetworkTask *task = [networking networkTaskWithRequest:request completionBlock:^(NSURLResponse *response, NSData *data, NSError *error) {
-    if (error) {
-      completionHandler(error, nil);
+    if (error || !response || !data) {
+      NSError *someError = nil;
+      if (error) {
+        someError = error;
+      } else if (!response) {
+        someError = RCTErrorWithMessage(@"Response metadata error");
+      } else {
+        someError = RCTErrorWithMessage(@"Unknown image download error");
+      }
+      completionHandler(someError, nil);
       [weakSelf dequeueTasks];
       return;
     }