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/strbuf.c

@@ -86,7 +86,7 @@ static void bubbleinsert(struct strbuf_node *node,
                 .c = c,
                 .child = node_child,
         };
-        int left = 0, right = node->children_count;
+        int left = 0, right = ASSERT_PTR(node)->children_count;
 
         while (right > left) {
                 int middle = (right + left) / 2 ;