[Issue #803] Only set image in UIImageView category if non-nil

Signed-off-by: Mattt Thompson <m@mattt.me>

AFNetworking/UIImageView+AFNetworking.m

@@ -117,7 +117,7 @@ static char kAFImageRequestOperationObjectKey;
             if ([urlRequest isEqual:[self.af_imageRequestOperation request]]) {
                 if (success) {
                     success(operation.request, operation.response, responseObject);
-                } else {
+                } else if (responseObject) {
                     self.image = responseObject;
                 }