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/devnum-util.c

@@ -18,6 +18,8 @@ int parse_devnum(const char *s, dev_t *ret) {
         size_t n;
         int r;
 
+        assert(ret);
+
         n = strspn(s, DIGITS);
         if (n == 0)
                 return -EINVAL;