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/uid-classification.c

@@ -25,6 +25,8 @@ static int parse_alloc_uid(const char *path, const char *name, const char *t, ui
         uid_t uid;
         int r;
 
+        assert(ret_uid);
+
         r = parse_uid(t, &uid);
         if (r < 0)
                 return log_debug_errno(r, "%s: failed to parse %s %s, ignoring: %m", path, name, t);