[io] avoid unnecessary heap allocation in TFile::MakeFree

[silverweed]

Micro-optimization in TFile::MakeFree()

Checklist:

• tested changes locally
• updated the docs (if necessary)

[pcanal]

Thanks.

[github-actions]

Test Results

    18 files      18 suites   5d 0h 12m 10s ⏱️
 2 683 tests  2 680 ✅ 0 💤 3 ❌
46 598 runs  46 594 ✅ 0 💤 4 ❌

For more details on these failures, see this check.

Results for commit fd5d522.