sendmmsg: fix a use-after-free case

Testing

Today I can't reproduce the problem locally; yesterday I could, but probably something on the live internet has changed. Still, respdiff-job-create --asan $commit does show improvement even today (repeatedly), against master (3d62d431) – shortlist.fwd-udp6-kresd.udp6.j384 gets fixed.

added 6 commits

• 1ca9045b...31ba97e0 - 3 commits from branch master
• b98f0c09 - daemon/udp_queue: add a ref-unref pair
• 298ade1e - daemon/worker: add assertion
• 7fcf1604 - NEWS for sendmmsg (preliminary text)

Compare with previous version

Statistically this improves things - crash in kresd goes away and it does not seem to affect the occasional leak detected by ASAN.

merged

mentioned in commit 440d1a80