lib/cache pkt_renew(): fix an edge-case bug

It could happen that this condition didn't get triggered, but the structures weren't completely clear. In particular, the current section could be past KNOT_ANSWER already. Let's be more conservative here; pkt_recycle() shouldn't be expensive.

I'm not sure why I only ran into this on the new-policy branch, but it really seems like bug here on master already.