many: more checks for pointer access without NULL check (#41370)

This is a followup for https://github.com/systemd/systemd/pull/41096
that makes more subsystems pass the new `check-pointer-deref` coccinelle
checks. See the individual commits.

My plan is to do a few more of these PRs until we have it all covered. I
could also do it in a single very big PR but I'm worried about a)
conflicts b) that its just too big/annoying to review. Only 7 dirs left
but some (like src/basic) are quite big (~50k loc) so those PRs will be
a bit bigger.