smbd: Fix use-after-free from exit_server_common()

We need to keep the smbXsrv_connection structures around until all
pending requests have had their chance to clean up behind them. If you
look at srv_send_smb(), it's exactly prepared already to just drop
anything on the floor when the transport has been declared dead:

	if (!NT_STATUS_IS_OK(xconn->transport.status)) {
		/*
		 * we're not supposed to do any io
		 */
		return true;
	}

Bug: https://bugzilla.samba.org/show_bug.cgi?id=14064

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Stefan Metzmacher <metze@samba.org>

Autobuild-User(master): Stefan Metzmacher <metze@samba.org>
Autobuild-Date(master): Thu Aug  1 15:39:13 UTC 2019 on sn-devel-184

(cherry picked from commit c226dc6e)
32 jobs for vl/bug-14064-v4.11 in 129 minutes and 53 seconds (queued for 2 seconds)