9pfs: fix crash when fsdev is missing

If the user passes -device virtio-9p without the corresponding -fsdev, QEMU
dereferences a NULL pointer and crashes.

This is a 2.8 regression introduced by commit 702dbcc274.

Signed-off-by: Greg Kurz <groug@kaod.org>
Reviewed-by: Li Qiang <liq3ea@gmail.com>
(cherry picked from commit f2b58c4375)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

hw/9pfs/9p.c

@@ -3521,7 +3521,7 @@ int v9fs_device_realize_common(V9fsState *s, Error **errp)
     rc = 0;
 out:
     if (rc) {
-        if (s->ops->cleanup && s->ctx.private) {
+        if (s->ops && s->ops->cleanup && s->ctx.private) {
             s->ops->cleanup(&s->ctx);
         }
         g_free(s->tag);