When we work on the last segment, the remaining length is zero,
but we still have a current pointer and length.

This was a particularly annoying glitch because I wrote almost
the same code for varnish-cache with the equivalent assertion in
the right place :(

Sorry

Ref https://github.com/varnishcache/varnish-cache/pull/4013/commits/8ec77190d91603c8f0dead0cee013e3c9ca8fa78#diff-f79cfeda8456789ae873270aefa58e8f1e94213ee16d32ea96b8db8a7013ebf8R790
Closes #34