Fix potential buffer overflow

[bellenot]

As reported here: https://github.com/root-project/root/security/code-scanning/1845

• Fixes Potential buffer overflow in TApplicationServer.cxx #22213
• Fixes https://github.com/root-project/root/security/code-scanning/1845

[github-actions]

Test Results

    22 files      22 suites   3d 11h 33m 6s ⏱️
 3 851 tests  3 850 ✅ 0 💤 1 ❌
76 953 runs  76 952 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 16f1100.