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/recurse-dir.c

@@ -16,6 +16,9 @@
 #define DEFAULT_RECURSION_MAX 100
 
 static int sort_func(struct dirent * const *a, struct dirent * const *b) {
+        assert(a);
+        assert(b);
+
         return strcmp((*a)->d_name, (*b)->d_name);
 }