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/in-addr-util.c

@@ -163,6 +163,8 @@ int in_addr_is_localhost_one(int family, const union in_addr_union *u) {
 }
 
 bool in6_addr_is_ipv4_mapped_address(const struct in6_addr *a) {
+        assert(a);
+
         return a->s6_addr32[0] == 0 &&
                 a->s6_addr32[1] == 0 &&
                 a->s6_addr32[2] == htobe32(UINT32_C(0x0000ffff));
@@ -728,6 +730,8 @@ int in4_addr_mask(struct in_addr *addr, unsigned char prefixlen) {
 int in6_addr_mask(struct in6_addr *addr, unsigned char prefixlen) {
         unsigned i;
 
+        assert(addr);
+
         for (i = 0; i < 16; i++) {
                 uint8_t mask;