virtio-net: only delete bh that existed

We delete without check whether it existed during exit. This will lead NULL
pointer deference since it was created conditionally depends on guest driver
status and features. So add a check of existence before trying to delete it.

Cc: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-id: 1383728288-28469-1-git-send-email-jasowang@redhat.com
Signed-off-by: Anthony Liguori <aliguori@amazon.com>

hw/net/virtio-net.c

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 22dbd05..ae51d96 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -1601,7 +1601,7 @@
         if (q->tx_timer) {
             timer_del(q->tx_timer);
             timer_free(q->tx_timer);
-        } else {
+        } else if (q->tx_bh) {
             qemu_bh_delete(q->tx_bh);
         }
     }