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)
Status Job ID Name Coverage
  Build
passed #264594193
shared docker
centos7-samba-o3

00:35:34

passed #264594164
shared docker
ctdb

00:39:05

passed #264594185
shared docker
debian9-samba-o3

00:34:30

passed #264594195
shared docker
fedora29-samba-o3

00:35:36

passed #264594196
shared docker
fedora30-samba-o3

00:33:53

passed #264594189
shared docker
opensuse150-samba-o3

00:41:14

passed #264594191
shared docker
opensuse151-samba-o3

00:40:49

passed #264594107
shared docker
others

00:14:42

passed #264594108
shared docker
samba

00:27:08

passed #264594174
samba-ci-private docker
samba-ad-dc-1

00:46:08

passed #264594130
shared docker
samba-ad-dc-2

01:17:13

passed #264594134
shared docker
samba-ad-dc-3

00:50:05

passed #264594139
shared docker
samba-ad-dc-4

00:55:38

passed #264679166
docker shared
samba-ad-dc-5

00:52:36

passed #264594151
shared docker
samba-ad-dc-6

00:40:51

passed #264594170
samba-ci-private docker
samba-ad-dc-backup

00:50:20

passed #264594169
shared docker
samba-ad-dc-ntvfs

00:45:47

passed #264594123
shared docker
samba-ad-member

00:25:04

passed #264594167
shared docker
samba-ctdb

00:21:59

passed #264594172
samba-ci-private docker
samba-fileserver

00:30:32

passed #264594156
shared docker
samba-libs

00:39:30

passed #264594110
shared docker
samba-none-env

00:56:19

passed #264594112
shared docker
samba-nopython

00:32:25

passed #264594113
shared docker
samba-nopython-py2

00:46:20

passed #264594176
samba-ci-private docker
samba-nt4

00:51:24

passed #264594178
samba-ci-private docker
samba-schemaupgrade

00:29:31

passed #264594158
shared docker
samba-static

00:49:13

passed #264594117
shared docker
samba-systemkrb5

00:20:07

passed #264594119
shared docker
samba-xc

00:11:26

passed #264594183
shared docker
ubuntu1604-samba-o3

00:34:16

passed #264594180
shared docker
ubuntu1804-samba-o3

00:34:23

failed #264594145
docker shared
samba-ad-dc-5

00:32:58