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/utf8.c

@@ -424,6 +424,7 @@ char* utf16_to_utf8(const char16_t *s, size_t length /* bytes! */) {
 }
 
 size_t utf16_encode_unichar(char16_t *out, char32_t c) {
+        assert(out);
 
         /* Note that this encodes as little-endian. */