While processing transmit descriptors, it could lead to an infinite
loop if 'bytes' was to become zero; Add a check to avoid it.

[The guest can force 'bytes' to 0 by setting the hdr_len and mss
descriptor fields to 0.
--Stefan]
Signed-off-by: NP J P <pjp@fedoraproject.org>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NThomas Huth <thuth@redhat.com>
Message-id: 1441383666-6590-1-git-send-email-stefanha@redhat.com