Fix one issue that when not authorized, there will call extra error completion handler

Author: dreampiggy

SDWebImagePhotosPlugin/Classes/SDImagePhotosLoader.m

@@ -130,6 +130,7 @@ - (BOOL)canRequestImageForURL:(NSURL *)url {
             NSError *error = [NSError errorWithDomain:SDWebImagePhotosErrorDomain code:SDWebImagePhotosErrorNotAuthorized userInfo:@{NSLocalizedDescriptionKey : @"Photos library access not authorized"}];
             completedBlock(nil, nil, error, YES);
         }
+        return nil;
     }
 
     PHFetchOptions *fetchOptions;