smbXsrv_{open,session,tcon}: protect smbXsrv_{open,session,tcon}_global_traverse_fn...

smbXsrv_{open,session,tcon}: protect smbXsrv_{open,session,tcon}_global_traverse_fn against invalid records

I saw systems with locking.tdb records being part of:
  ctdb catdb smbXsrv_tcon_global.tdb

It's yet unknown how that happened, but we should not panic in srvsvc_*
calls because the info0 pointer was NULL.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14752

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Tue Jul  6 11:08:43 UTC 2021 on sn-devel-184

(cherry picked from commit 00bab5b3c821f272153a25ded9743460887a7907)

Autobuild-User(v4-13-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-13-test): Tue Jul 13 13:18:20 UTC 2021 on sn-devel-184