Fix nil pointer panic when using MD5 hash comparison on FAT32/exFAT drives

[Copilot]

This PR fixes an immediate runtime panic that occurs when using the --compare-hash=MD5 flag with sync operations on FAT32/exFAT formatted drives on Windows.

Problem

When running commands like:

azcopy sync "D:\HashingTest" $destination --put-md5 --compare-hash=MD5

On FAT32/exFAT drives, users encountered an immediate panic:

panic: runtime error: invalid memory address or nil pointer dereference
[signal 0xc0000005 code=0x0 addr=0x8 pc=0x13b2d32]

goroutine 1 [running]:
github.com/Azure/azure-storage-azcopy/v10/cmd.(*localTraverser).IsDirectory(0x0?, 0x0?)

Root Cause

The issue occurred because:

1. Hash adapter initialization failure: On Windows, AzCopy defaults to using Alternate Data Streams (ADS) for hash storage, but FAT32/exFAT filesystems don't support ADS
2. Error swallowing: In zc_enumerator.go line 463, the error from newLocalTraverser() was being ignored using output, _ = newLocalTraverser(...)
3. Nil pointer dereference: This resulted in a nil traverser that caused a panic when IsDirectory() was called later in the sync process

Solution

The fix is minimal and surgical - properly handle the error from newLocalTraverser() instead of ignoring it:

Before:

output, _ = newLocalTraverser(resource.ValueLocal(), ctx, opts)

After:

output, err = newLocalTraverser(resource.ValueLocal(), ctx, opts)
if err != nil {
    return nil, err
}

Impact

• Eliminates panic: Users no longer experience runtime crashes when using hash comparison on unsupported filesystems
• Provides clear error messages: Instead of a panic, users receive informative error messages about hash adapter initialization failures
• Preserves existing functionality: All existing behavior remains unchanged for supported scenarios
• Enables proper error handling: Users can now handle the error gracefully and choose alternative approaches

This change allows users to receive proper feedback about filesystem compatibility issues rather than experiencing unexpected crashes, enabling them to use alternative hash storage modes or different filesystems as needed.

Fixes #3221.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• fake.blob.core.windows.net
  • Triggering command: ./azcopy/azure-storage-azcopy sync /tmp/hash_init_test_1758259955 REDACTED --compare-hash=MD5 --dry-run (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

@vibhansa-msft 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.