net_queue_manager: fix issue in de-fragmentation
net_queue_manager: fix issue in de-fragmentation

Some de-fragmented packets may cause the ethernet server will crash.
I suggest attached patch as fix.

to recreate segfault:
ping barrelfish.host -s 1473

linusyang/barrelfish,mslovy/barrelfish,BarrelfishOS/barrelfish,utsav2601/cmpe295A,kishoredbn/barrelfish,linusyang/barrelfish,BarrelfishOS/barrelfish,mslovy/barrelfish,kishoredbn/barrelfish,utsav2601/cmpe295A,linusyang/barrelfish,BarrelfishOS/barrelfish,utsav2601/cmpe295A,utsav2601/cmpe295A,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,linusyang/barrelfish,8l/barrelfish,mslovy/barrelfish,8l/barrelfish,kishoredbn/barrelfish,kishoredbn/barrelfish,mslovy/barrelfish,kishoredbn/barrelfish,linusyang/barrelfish,BarrelfishOS/barrelfish,8l/barrelfish,8l/barrelfish,8l/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,BarrelfishOS/barrelfish,mslovy/barrelfish,mslovy/barrelfish,8l/barrelfish,utsav2601/cmpe295A,utsav2601/cmpe295A,kishoredbn/barrelfish