libvector/neta: fix memory leaks

[nilason]

Fix some memory leaks in lib/vector/neta exposed by static analysis.

[echoix]

The three failing tests seem to be failing often for Python 3.12. They timeout.
I have two ideas. If these tests use multiple cores, since multiple test runners are launched in parallel, we might be having too much work launched. In January 2024, GitHub finished upgrading the runners to offer 4 cpu cores https://github.blog/2024-01-17-github-hosted-runners-double-the-power-for-open-source/

My second idea, is the deprecation warning shown, that the forks used might cause deadlocks in the child, and we might be in that situation.

[echoix]

A couple weeks ago, I tried using the number of logical cores to run tests, but got more timeouts than that (or timed out more easily).

https://pytest-xdist.readthedocs.io/en/latest/distribution.html#running-tests-across-multiple-cpus

So I didn't continue with that. We are thus using 2 runners for 4 cpu.