migration/multifd: fix nullptr access in terminating multifd threads

One multifd channel will shutdown all the other multifd's IOChannel when it
fails to receive an IOChannel. In this senario, if some multifds had not
received its IOChannel yet, it would try to shutdown its IOChannel which could
cause nullptr access at qio_channel_shutdown.

Here is the coredump stack:
    #0  object_get_class (obj=obj@entry=0x0) at qom/object.c:908
    kata-containers#1  0x00005563fdbb8f4a in qio_channel_shutdown (ioc=0x0, how=QIO_CHANNEL_SHUTDOWN_BOTH, errp=0x0) at io/channel.c:355
    kata-containers#2  0x00005563fd7b4c5f in multifd_recv_terminate_threads (err=<optimized out>) at migration/ram.c:1280
    kata-containers#3  0x00005563fd7bc019 in multifd_recv_new_channel (ioc=ioc@entry=0x556400255610, errp=errp@entry=0x7ffec07dce00) at migration/ram.c:1478
    kata-containers#4  0x00005563fda82177 in migration_ioc_process_incoming (ioc=ioc@entry=0x556400255610, errp=errp@entry=0x7ffec07dce30) at migration/migration.c:605
    kata-containers#5  0x00005563fda8567d in migration_channel_process_incoming (ioc=0x556400255610) at migration/channel.c:44
    kata-containers#6  0x00005563fda83ee0 in socket_accept_incoming_migration (listener=0x5563fff6b920, cioc=0x556400255610, opaque=<optimized out>) at migration/socket.c:166
    kata-containers#7  0x00005563fdbc25cd in qio_net_listener_channel_func (ioc=<optimized out>, condition=<optimized out>, opaque=<optimized out>) at io/net-listener.c:54
    kata-containers#8  0x00007f895b6fe9a9 in g_main_context_dispatch () from /usr/lib64/libglib-2.0.so.0
    kata-containers#9  0x00005563fdc18136 in glib_pollfds_poll () at util/main-loop.c:218
    kata-containers#10 0x00005563fdc181b5 in os_host_main_loop_wait (timeout=1000000000) at util/main-loop.c:241
    qemu#11 0x00005563fdc183a2 in main_loop_wait (nonblocking=nonblocking@entry=0) at util/main-loop.c:517
    qemu#12 0x00005563fd8edb37 in main_loop () at vl.c:1791
    qemu#13 0x00005563fd74fd45 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4473

To fix it up, let's check p->c before calling qio_channel_shutdown.

Signed-off-by: Jiahui Cen <[email protected]>
Signed-off-by: Ying Fang <[email protected]>
Reviewed-by: Juan Quintela <[email protected]>
Signed-off-by: Juan Quintela <[email protected]>

migration/ram.c

@@ -1308,7 +1308,9 @@ static void multifd_recv_terminate_threads(Error *err)
            - normal quit, i.e. everything went fine, just finished
            - error quit: We close the channels so the channel threads
              finish the qio_channel_read_all_eof() */
-        qio_channel_shutdown(p->c, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
+        if (p->c) {
+            qio_channel_shutdown(p->c, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
+        }
         qemu_mutex_unlock(&p->mutex);
     }
 }