demux: fix a specific gcc 4.8 warning that may hint to mis-optimized code

CC      demux/demux.o
demux/demux.c: In function 'demuxer_switch_track':
demux/demux.c:1241:29: warning: array subscript is above array bounds [-Warray-bounds]
     int new_id = demuxer->ds[type]->id;
                                  ^