CVE-2013-6401: Change hash function, randomize hashes

Thanks to Florian Weimer and Eric Sesterhenn for reporting, reviewing
and testing.
static int json_string_equal(json_t *string1, json_t *string2)
{
    return strcmp(json_string_value(string1), json_string_value(string2)) == 0;
}

static int json_string_equal(json_t *string1, json_t *string2)
{
    return strcmp(json_string_value(string1), json_string_value(string2)) == 0;
}
