Consider this test program:

  #include <stdio.h>
  #include <locale.h>
  #include <fnmatch.h>
  int main(int argc, char *argv[]) {
    setlocale(LC_ALL, "");
    if (argc != 3) {
      fprintf(stderr, "usage: fnmatch <pattern> <string>\n");
      return 2;
    }
    return !!fnmatch(argv[1], argv[2], 0);
  }

When called as

  export LC_ALL=en_US.UTF-8
  ./fnmatch $'[[.L\u00B7.]]' . # [[.L·.]]

This results in a segmentation fault in internal_fnwmatch.

Seen on glibc 2.32 built with GCC 10.2.0, also reproducible with the libc6-2.31-0ubuntu9 provided by Ubuntu 20.04 for amd64.

(I think there are a few more bugs in there that do not result in a crash, but do result in a wrong return value. Should I include the details in here or report that separately?)