Mozilla #516847 - calc gp for duplicate frames

Theora encodes duplicate frames as zero-length packets, so
increment the calculated granulepos correctly for these.

Patch by Matthew Gregan

src/liboggz/oggz_auto.c

@@ -568,7 +568,7 @@ auto_calc_theora(ogg_int64_t now, oggz_stream_t *stream, ogg_packet *op) {
   unsigned char first_byte;
   auto_calc_theora_info_t *info;
 
-  first_byte = op->packet[0];
+  first_byte = op->bytes == 0 ? 0x40 : op->packet[0];
 
   info = (auto_calc_theora_info_t *)stream->calculate_data;