The function was performing pointer arithmetic on potentially NULL pointer
returned by strrchr(), and comparing NULL with fn pointer, both of which
are undefined behavior.

Fix:
- Check if strrchr() returns NULL before adding 1
- Use ternary operator: 'off ? off + 1 : fn'
- Avoids both NULL pointer arithmetic and NULL comparison UB

This prevents crashes in debug builds as reported in the issue.

Fixes wasm3#505
Credits to @InBetweenNames for identifying the issue and suggesting the fix