libcli/smb: only check the SMB2 session setup signature if required and valid

libcli/smb: only check the SMB2 session setup signature if required and valid

This is an update to commit af290a03cef63c3b08446c1980de064a3b1c8804
that skips the scary debug messages.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=10146

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Wed Sep 18 04:46:00 CEST 2013 on sn-devel-104
(cherry picked from commit 4879d0810a2ad741e32ad174a7a14cd35521aeaf)