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/mempool.c

@@ -65,6 +65,8 @@ void* mempool_alloc_tile(struct mempool *mp) {
 void* mempool_alloc0_tile(struct mempool *mp) {
         void *p;
 
+        assert(mp);
+
         p = mempool_alloc_tile(mp);
         if (p)
                 memzero(p, mp->tile_size);