avconv: do not free muxed packet on streamcopy.

The packet belongs to lavf, the caller must not free it or access it in
any other way.
No change in practice, since destruct is set to NULL anyway.

avconv.c

@@ -1045,7 +1045,6 @@ static void do_streamcopy(InputStream *ist, OutputStream *ost, const AVPacket *p
 
     write_frame(of->ctx, &opkt, ost);
     ost->st->codec->frame_number++;
-    av_free_packet(&opkt);
 }
 
 static void rate_emu_sleep(InputStream *ist)