Fix signedness bug.
Fix signedness bug.

Thanks to falkevik for reporting the issue and suggesting a
fix. This fixes issue #182.

faithfracture/usrsctp,faithfracture/usrsctp,CoolmanCZ/usrsctp,sctplab/usrsctp,sctplab/usrsctp,sctplab/usrsctp,JlnWntr/usrsctp,CoolmanCZ/usrsctp,weinrank/usrsctp,weinrank/usrsctp,faithfracture/usrsctp,CoolmanCZ/usrsctp,CoolmanCZ/usrsctp,JlnWntr/usrsctp,JlnWntr/usrsctp,weinrank/usrsctp