virtio_scsi: verify if queue is broken after virtqueue_get_buf()

If virtqueue_get_buf() returned with a NULL pointer avoid a possibly
endless loop by checking for a broken virtqueue.

Signed-off-by: Heinz Graalfs <graalfs@linux.vnet.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>