Improve error checking in labelsToString.
Improve error checking in labelsToString.

dxq-git/polipo,nxtreaming/chumen,akkakks/polipo,fqtools/polipo,berkley4/polipo,lkraider/polipo,nxtreaming/chumen,feijilei/polipo,fqtools/polipo,galeksandrp/polipo,fqtools/polipo,TierraDelFuego/opilop,feijilei/polipo,ajmgh/polipo,akkakks/polipo,Psiphon-Inc/polipo,lkraider/polipo,akkakks/polipo,nxtreaming/chumen,clowwindy/polipo,berkley4/polipo,ssb22/polipo2,lkraider/polipo,galeksandrp/polipo,dxq-git/polipo,galeksandrp/polipo,jech/polipo,dxq-git/polipo,ssb22/polipo2,jech/polipo,feijilei/polipo,jech/polipo,berkley4/polipo,ssb22/polipo2