s3:smbd: Do not crash if we fail to init the session table

s3:smbd: Do not crash if we fail to init the session table

This should the following segfault with SMB1:

  #6  sig_fault (sig=<optimized out>) at ../lib/util/fault.c:94
  #7  <signal handler called>
  #8  smbXsrv_session_create (conn=conn@entry=0x5654d3512af0, now=now@entry=131594481900356690, _session=_session@entry=0x7ffc93a778e8)
      at ../source3/smbd/smbXsrv_session.c:1212
  #9  0x00007f7618aa21ef in reply_sesssetup_and_X (req=req@entry=0x5654d35174b0) at ../source3/smbd/sesssetup.c:961
  #10 0x00007f7618ae17b0 in switch_message (type=<optimized out>, req=req@entry=0x5654d35174b0) at ../source3/smbd/process.c:1726
  #11 0x00007f7618ae3550 in construct_reply (deferred_pcd=0x0, encrypted=false, seqnum=0, unread_bytes=0, size=140, inbuf=0x0, xconn=0x5654d35146d0)
      at ../source3/smbd/process.c:1762
  #12 process_smb (xconn=xconn@entry=0x5654d3512af0, inbuf=<optimized out>, nread=140, unread_bytes=0, seqnum=0, encrypted=<optimized out>,
      deferred_pcd=deferred_pcd@entry=0x0) at ../source3/smbd/process.c:2008
  #13 0x00007f7618ae4c41 in smbd_server_connection_read_handler (xconn=0x5654d3512af0, fd=40) at ../source3/smbd/process.c:2608
  #14 0x00007f761587eedb in epoll_event_loop_once () from /lib64/libtevent.so.0

Inspection the core shows that:
  conn->client-session_table is NULL
  conn->protocol is PROTOCOL_NONE

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13315

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>