Fix compiler warning: specified bound 4096 equals destination size

[kratz00]

Fixes the following compiler warning:

[6/13] Building C object lib/CMakeFiles/libunshield.dir/helper.c.o
/home/runner/work/unshield/unshield/lib/helper.c: In function ‘unshield_get_base_directory_name’:
/home/runner/work/unshield/unshield/lib/helper.c:50:9: warning: ‘__builtin_strncpy’ specified bound 4096 equals destination size [-Wstringop-truncation]
   50 |         strncpy(dirname, unshield->filename_pattern, path_max);
      |         ^

I guess it does not make sense to continue, if dirname is too small.
So I refactored the code and used the new UNSHIELD_ASSERT 😄