Tighter constraints on the maximum length of the filename handed over

Tighter constraints on the maximum length of the filename handed over
to sqlite3_load_extension(), due to
[forum:/forumpost/a43074729e|forum post a43074729e].  This is a
follow-on to [01f3877c7172d522] and
[forum:/forumpost/08a0d6d9bf|forum post 08a0d6d9bf].

FossilOrigin-Name: 9f351bdee2a09a4419bb8256a13d1f757b3e00ec26a445523f224fc56d9ae26d