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: NLi Qiang <liqiang6-s@360.cn>
Signed-off-by: NPrasad J Pandit <pjp@fedoraproject.org>
Reviewed-by: NDmitry Fleytman <dmitry@daynix.com>
CC: qemu-stable@nongnu.org
Signed-off-by: NJason Wang <jasowang@redhat.com>

hw/net/net_tx_pkt.c

@@ -590,7 +590,7 @@ static bool net_tx_pkt_do_sw_fragmentation(struct NetTxPkt *pkt,
 
         fragment_offset += fragment_len;
 
-    } while (more_frags);
+    } while (fragment_len && more_frags);
 
     return true;
 }