packet: Fix ipv6.hdr() with string payload
packet: Fix ipv6.hdr() with string payload

Previously, a string payload would cause an exception.

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