Fix crash on nick change
Fix crash on nick change

Elemental-IRCd/irc,SeanThomasWilliams/go-ircevent,oniichaNj/go-ircevent,pratikmallya/go-ircevent,DanielOaks/girc-go,alaska/go-ircevent,phone/go-ircevent,iwanbk/ogric,dainis/go-ircevent,HenrySlawniak/go-ircevent