net: check fragment length during fragmentation

Network transport abstraction layer supports packet fragmentation.
While fragmenting a packet, it checks for more fragments from
packet length and current fragment length. It is susceptible
to an infinite loop, if the current fragment length is zero.
Add check to avoid it.

Reported-by: Li Qiang <liqiang6-s@360.cn>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
CC: qemu-stable@nongnu.org
Signed-off-by: Jason Wang <jasowang@redhat.com>