demux: actually initialise packet buffer when creating new packet

When I refactored the code here to stop using stack allocated
AVPackets, I forgot to assign `dp->buffer` after initialising the
AVPacket.

Fixes #11097

demux/packet.c

@@ -135,6 +135,7 @@ struct demux_packet *new_demux_packet(size_t len)
         talloc_free(dp);
         return NULL;
     }
+    dp->buffer = dp->avpacket->data;
     dp->len = len;
     return dp;
 }