The DBG_DEBUG() in ad_open() dereferences smb_fname, which can be NULL
when ad_fget() calls ad_get_internal(). Add an SMB_ASSERT() guard in
ad_fget() to catch this early.
Signed-off-by: Anoop C S <anoopcs@samba.org> Reviewed-by: Martin Schwenke <martin@meltin.net>