lib: avoid format truncation

lib: avoid format truncation

    commonio.c: In function 'commonio_unlock':
    commonio.c:487:49: warning: '.lock' directive output may be truncated writing 5 bytes into a region of size between 1 and 1024 [-Wformat-truncation=]
      487 |                 snprintf (lock, sizeof lock, "%s.lock", db->filename);
          |                                                 ^~~~~
    commonio.c:487:17: note: 'snprintf' output between 6 and 1029 bytes into a destination of size 1024
      487 |                 snprintf (lock, sizeof lock, "%s.lock", db->filename);
          |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~