Check return value of if_indextoname().
Check return value of if_indextoname().

This should fix issue #157.

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