fix(strbuf): resolve potential use-after-free and UB in ffStrbufSetNS

[fam007e]

Summary

This PR fixes a "Potential use after free" security alert flagged by CodeQL and addresses Undefined Behavior (UB) when setting a buffer to a substring of itself.

Related issue (required for new logos for new distros)

Fixes a CodeQL security alert (no public issue was found).

Changes

• Use-After-Free Fix: Modified ffStrbufSetNS to allocate and copy into a new buffer before freeing the old one. This ensures that if the source value points into the current buffer, it remains valid during the copy even if reallocation is required.
• Overlap Safety: Replaced memcpy with memmove in the non-reallocating path to safely handle cases where value points to an overlapping memory region within the same buffer.

Checklist

• I have tested my changes locally. (Verified with fastfetch-test-strbuf)