basic: add assert() when doing pointer deref

Lennart reminded me in [1] that we need to add assert() in functions
that do pointer access. For the simple `*p` pointer dereferences
we even have an automatic coccinelle script that ensures that as
part of the automatic code checks.

However for deref in the `p->` style this is not supported right
now and adding it to coccinelle is hard because its too slow for
this kind of check. So I created a (slightly messy) tree-sitter
python script to see how many asserts we are currently missing.

This commit is the result of running it over the `src/basic`
dir and fixing the flagged issues. I plan to tidy it up and
add it to the checks too but this is orthogonal to this commit.

[1] https://github.com/systemd/systemd/pull/42360#discussion_r3426964562

src/basic/sysctl-util.c

@@ -16,6 +16,8 @@
 char* sysctl_normalize(char *s) {
         char *n;
 
+        assert(s);
+
         n = strpbrk(s, "/.");
 
         /* If the first separator is a slash, the path is