many: use POINTER_MAY_BE_NULL in inner sort _cmp funcs

Some functions are in the hot path of a sort. While it is more
correct to also use assert() here for the passed pointers it is
also a runtime cost that should never be needed when e.g. qsort()
is used as it will always pass non-NULL pointers.

So for these critical functions use POINTER_MAY_BE_NULL() which
is compiled out and add a comment to them. Not all _compare()
functions are changed, it was a bit of a judgement call because
some (like ether_addr_compare) are not called with that much data.

But happy to get advise here and tweak accordingly.

src/basic/sort-util.c

@@ -65,15 +65,19 @@ 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);
+        /* This is called from qsort()s inner loops. Correctly implemented qsort will never pass NULL so we
+           just suppress the check via POINTER_MAY_BE_NULL instead of assert() to avoid the runtime cost. */
+        POINTER_MAY_BE_NULL(a);
+        POINTER_MAY_BE_NULL(b);
 
         return CMP(*a, *b);
 }
 
 int cmp_uint16(const uint16_t *a, const uint16_t *b) {
-        assert(a);
-        assert(b);
+        /* This is called from qsort()s inner loops. Correctly implemented qsort will never pass NULL so we
+           just suppress the check via POINTER_MAY_BE_NULL instead of assert() to avoid the runtime cost. */
+        POINTER_MAY_BE_NULL(a);
+        POINTER_MAY_BE_NULL(b);
 
         return CMP(*a, *b);
 }