fix(storage): bug when getting the root ref of a bucket (#3455)

A bug crashes the app when trying to list objects, root urls don't have a "/" so rangeOfString gives location == NSNotFound, if it goes unchecked rangeOfSlash.location + 5 creates range outside the bounds

Co-authored-by: Mike Hardy <github@mikehardy.net>

packages/storage/ios/RNFBStorage/RNFBStorageModule.m

@@ -517,7 +517,11 @@ - (void)addUploadTaskObservers:(FIRStorageUploadTask *)uploadTask appDisplayName
 
 - (FIRStorageReference *)getReferenceFromUrl:(NSString *)url app:(FIRApp *)firebaseApp {
   NSString *pathWithBucketName = [url substringWithRange:NSMakeRange(5, [url length] - 5)];
-  NSString *bucket = [url substringWithRange:NSMakeRange(0, [pathWithBucketName rangeOfString:@"/"].location + 5)];
+  NSString *bucket = url;
+  NSRange rangeOfSlash = [pathWithBucketName rangeOfString:@"/"];
+  if (rangeOfSlash.location != NSNotFound) {
+    bucket = [url substringWithRange:NSMakeRange(0, rangeOfSlash.location + 5)];
+  }
   return [[FIRStorage storageForApp:firebaseApp URL:bucket] referenceForURL:url];
 }