Unchecked lower bound warning on `regex_traits_defaults.hpp::get_default_error_string()`

Flagged by MSVC as [C33010](https://learn.microsoft.com/en-us/cpp/code-quality/c33010?view=msvc-170) (there might be others in the same file).

Start in regex_traits_defaults.hpp at get_default_error_string() and read the MSVC C33010 warning documentation linked in the issue. Inspect the lower-bound handling there and check whether other warnings occur in the same file; done means the warning is addressed without introducing new diagnostics.

Written by the indexing model from the issue text.