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/sort-util.c

@@ -65,9 +65,15 @@ void qsort_r_safe(void *base, size_t nmemb, size_t size, comparison_userdata_fn_
 }
 
 int cmp_int(const int *a, const int *b) {
+        assert(a);
+        assert(b);
+
         return CMP(*a, *b);
 }
 
 int cmp_uint16(const uint16_t *a, const uint16_t *b) {
+        assert(a);
+        assert(b);
+
         return CMP(*a, *b);
 }