[io] Fix TFile::WalkTKeys mishandling strings longer than 127 characters

[silverweed]

Using a signed char is incorrect, as strings in root files can be up to 255 characters long (as far as I understand).

Checklist:

• [x] tested changes locally
• [ ] updated the docs (if necessary)

[github-actions[bot]]

Test Results

    19 files      19 suites   3d 16h 15m 17s ⏱️  2 811 tests  2 809 ✅ 0 💤 2 ❌ 51 909 runs  51 907 ✅ 0 💤 2 ❌

For more details on these failures, see this check.

Results for commit ca2300f3.

:recycle: This comment has been updated with latest results.

[silverweed]

@pcanal can this be merged?