basic: make check-pointer-deref clean

Add the needed assert changes to make the code clean
for the new check-pointer-deref script.

Note that for `_cmp()` functions its a bit of a question
if we want `assert()` or `POINTER_MAY_BE_NULL()` here. The
former is more correct but it adds a (small) runtime hit. If
we use POINTER_MAY_BE_NULL() it is compiled out but strictly
speaking it is wrong (so at least we would need a comment).

src/basic/in-addr-util.c

@@ -841,6 +841,8 @@ int in_addr_parse_prefixlen(int family, const char *p, unsigned char *ret) {
         uint8_t u;
         int r;
 
+        assert(ret);
+
         if (!IN_SET(family, AF_INET, AF_INET6))
                 return -EAFNOSUPPORT;