demux: new packet should not point to source buffer when copying data

In `new_demux_packet_from`, we initialise a new packet and allocate a
buffer which we copy the source data into. But I was then assigning
the original source pointer as the packet's buffer, rather than keeping
the newly allocated one. Whoops.

demux/packet.c

@@ -120,7 +120,6 @@ struct demux_packet *new_demux_packet_from(void *data, size_t len)
     if (!dp)
         return NULL;
     memcpy(dp->avpacket->data, data, len);
-    dp->buffer = data;
     return dp;
 }