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/string-table.c

@@ -41,6 +41,8 @@ ssize_t string_table_lookup_from_string_with_boolean(const char * const *table,
 int string_table_lookup_to_string_fallback(const char * const *table, size_t len, ssize_t i, size_t max, char **ret) {
         char *s;
 
+        assert(ret);
+
         if (i < 0 || i > (ssize_t) max)
                 return -ERANGE;