virtio-scsi: don't crash without a valid device

Make sure that we actually have a device when checking the aio
context. Otherwise guests could trigger QEMU crashes.
Signed-off-by: N"Eugene (jno) Dvurechenski" <jno@linux.vnet.ibm.com>
Reviewed-by: NDavid Hildenbrand <dahi@linux.vnet.ibm.com>
Message-Id: <1448549135-6582-2-git-send-email-jno@linux.vnet.ibm.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

hw/scsi/virtio-scsi.c

@@ -250,7 +250,7 @@ static int virtio_scsi_do_tmf(VirtIOSCSI *s, VirtIOSCSIReq *req)
     int target;
     int ret = 0;
 
-    if (s->dataplane_started) {
+    if (s->dataplane_started && d) {
         assert(blk_get_aio_context(d->conf.blk) == s->ctx);
     }
     /* Here VIRTIO_SCSI_S_OK means "FUNCTION COMPLETE".  */