Fix exception on link down
Fix exception on link down

This closes issue #17.

Topology is getting a huge makeover and this will no longer be
relevant, but might as well address this bug in the meantime.

andiwundsam/_of_normalize,kulawczukmarcin/mypox,xAKLx/pox,PrincetonUniversity/pox,chenyuntc/pox,jacobq/csci5221-viro-project,VamsikrishnaNallabothu/pox,denovogroup/pox,VamsikrishnaNallabothu/pox,jacobq/csci5221-viro-project,kulawczukmarcin/mypox,kavitshah8/SDNDeveloper,diogommartins/pox,diogommartins/pox,jacobq/csci5221-viro-project,PrincetonUniversity/pox,andiwundsam/_of_normalize,MurphyMc/pox,waltznetworks/pox,adusia/pox,kpengboy/pox-exercise,chenyuntc/pox,carlye566/IoT-POX,denovogroup/pox,pthien92/sdn,adusia/pox,xAKLx/pox,VamsikrishnaNallabothu/pox,kavitshah8/SDNDeveloper,carlye566/IoT-POX,kavitshah8/SDNDeveloper,diogommartins/pox,MurphyMc/pox,carlye566/IoT-POX,MurphyMc/pox,kavitshah8/SDNDeveloper,kpengboy/pox-exercise,xAKLx/pox,pthien92/sdn,noxrepo/pox,chenyuntc/pox,denovogroup/pox,denovogroup/pox,pthien92/sdn,noxrepo/pox,andiwundsam/_of_normalize,jacobq/csci5221-viro-project,kulawczukmarcin/mypox,PrincetonUniversity/pox,pthien92/sdn,VamsikrishnaNallabothu/pox,noxrepo/pox,waltznetworks/pox,MurphyMc/pox,denovogroup/pox,diogommartins/pox,waltznetworks/pox,PrincetonUniversity/pox,adusia/pox,PrincetonUniversity/pox,noxrepo/pox,adusia/pox,carlye566/IoT-POX,diogommartins/pox,xAKLx/pox,kpengboy/pox-exercise,VamsikrishnaNallabothu/pox,xAKLx/pox,chenyuntc/pox,andiwundsam/_of_normalize,kpengboy/pox-exercise,pthien92/sdn,adusia/pox,waltznetworks/pox,kpengboy/pox-exercise,jacobq/csci5221-viro-project,kulawczukmarcin/mypox,chenyuntc/pox,carlye566/IoT-POX,kulawczukmarcin/mypox,waltznetworks/pox,MurphyMc/pox