virtio-net: fix handling of bad buffers

When we discard a bad buffer, we don't advance to the next one, so we'll free
it again on the next pass, resulting in a double-free.

Tested-by: Asias He <asias@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>