Potential buffer overflow in TString.cxx

### Check duplicate issues.

- [x] Checked for duplicates

### Description

See https://github.com/root-project/root/security/code-scanning/1843

### Reproducer

See also https://github.com/root-project/root/security/code-scanning/1843

### ROOT version

master

### Installation method

N/A

### Operating system

All

### Additional context

_No response_