Tolerate READ_STOP_OK for buffered packets

This fixes an issue with some Kate tracks not playing with liboggplay,
reported by both Viktor Gal and ogg.k.ogg.k

Adapated from patch by ogg.k.ogg.k

src/liboggz/oggz_read.c

@@ -306,14 +306,14 @@ oggz_read_deliver_packet(void *elem) {
 
   if (p->stream->read_packet) {
     if ((cb_ret = p->stream->read_packet(p->oggz, &(p->packet), p->serialno, 
-			       p->stream->read_user_data)) != 0) {
+			       p->stream->read_user_data)) < 0) {
       p->oggz->cb_next = cb_ret;
       if (cb_ret == -1)
         return DLIST_ITER_ERROR;
     }
   } else if (p->reader->read_packet) {
     if ((cb_ret = p->reader->read_packet(p->oggz, &(p->packet), p->serialno, 
-			       p->reader->read_user_data)) != 0) {
+			       p->reader->read_user_data)) < 0) {
       p->oggz->cb_next = cb_ret;
       if (cb_ret == -1)
         return DLIST_ITER_ERROR;