Fix potential buffer overflow in TString

[bellenot]

As reported here: https://github.com/root-project/root/security/code-scanning/1843

• Fixes Potential buffer overflow in TString.cxx #22218
• Fixes https://github.com/root-project/root/security/code-scanning/1843

[github-actions]

Test Results

    22 files      22 suites   3d 10h 24m 30s ⏱️
 3 848 tests  3 796 ✅ 0 💤 52 ❌
76 009 runs  75 957 ✅ 0 💤 52 ❌

For more details on these failures, see this check.

Results for commit f50062d.