Fix non-portable use of NULL.
Fix non-portable use of NULL.

dxq-git/polipo,jech/polipo,ssb22/polipo2,ssb22/polipo2,feijilei/polipo,galeksandrp/polipo,jech/polipo,jech/polipo,fqtools/polipo,feijilei/polipo,fqtools/polipo,fqtools/polipo,berkley4/polipo,dxq-git/polipo,berkley4/polipo,feijilei/polipo,dxq-git/polipo,berkley4/polipo,galeksandrp/polipo,ssb22/polipo2,galeksandrp/polipo