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

Thanks to Florian Weimer and Eric Sesterhenn for reporting, reviewing
and testing.
static JSON_INLINE int bucket_is_empty(hashtable_t *hashtable, bucket_t *bucket)
{
    return bucket->first == &hashtable->list && bucket->first == bucket->last;
}

static JSON_INLINE int bucket_is_empty(hashtable_t *hashtable, bucket_t *bucket)
{
    return bucket->first == &hashtable->list && bucket->first == bucket->last;
}
