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/extract-word.c

@@ -204,6 +204,9 @@ int extract_first_word_and_warn(
         const char *save;
         int r;
 
+        assert(p);
+        assert(ret);
+
         save = *p;
         r = extract_first_word(p, ret, separators, flags);
         if (r >= 0)