Fix Storage usage of invalid pointers. (#1574)

* Move the usage of the const char* path into the original thread.

* Add readme.

* Add link to issue.

* Add namespace to readme note.

Author: jonsimantov

release_build_files/readme.md

@@ -631,6 +631,12 @@ workflow use only during the development of your app, not for publicly shipping
 code.
 
 ## Release Notes
+### Upcoming Release
+-   Changes
+    - Storage (iOS): Fix invalid pointer in `StorageReference::GetFile()` when
+      running in a secondary thread
+      ([#1570](https://github.com/firebase/firebase-cpp-sdk/issues/1570)).
+
 ### 11.10.0
 -   Changes
     - General (Android): Update to Firebase Android BoM version 32.8.1.

storage/src/ios/storage_reference_ios.mm

@@ -142,11 +142,12 @@
   // Cache a copy of the impl and storage, in case this is destroyed before the thread runs.
   FIRStorageReference* my_impl = impl();
   StorageInternal* storage = storage_;
+  NSURL* local_file_url = [NSURL URLWithString:@(path)];
   util::DispatchAsyncSafeMainQueue(^() {
       FIRStorageDownloadTask *download_task;
       {
         MutexLock mutex(controller_init_mutex_);
-        download_task = [my_impl writeToFile:[NSURL URLWithString:@(path)] completion:completion];
+        download_task = [my_impl writeToFile:local_file_url completion:completion];
         local_controller->AssignTask(storage, download_task);
       }
       if (listener) {