Summary

When batch_mode=async is configured, batcher.Commit returns immediately with a nil *api.MediaItem result (the actual commit happens later in a background goroutine). The old code passed this nil directly to o.setMetaData(info), which dereferenced it and panicked:

runtime error: invalid memory address or nil pointer dereference

This PR adds a defensive nil guard to setMetaData and updates Update to use the local size when the API response is deferred, preventing the crash.

Technical Details

• Adds a nil guard in setMetaData(info) to handle deferred results from async batcher commits.
• Uses src.Size() for o.bytes in Update when info == nil (so caching layers like the hasher overlay can immediately record the correct fingerprint).

Automated Tests

Adds the TestAsyncBatchModePanic regression test in backend/googlephotos/googlephotos_workaround_test.go verifying the fix under async mode.

Manual Verification

Run an upload/overwrite in async mode:

./rclone copy test_photo.jpg gphotos:album/rclone_PR4_Test \
  --gphotos-batch-mode=async --gphotos-batch-size=1 -v

Before the fix, this crashed immediately. After the fix, it succeeds without panic.

Dependencies (gh-stack)

This PR is independent and has no dependencies.

Was the change discussed in an issue or in the forum before?

As far as I know, this was not discussed previously.

Checklist

• I have read the contribution guidelines.
• I have added tests for all changes in this PR if appropriate.
• I have added documentation for the changes if appropriate.
• All commit messages are in house style.
• I'm done, this Pull Request is ready for review :-)