demux: mp4: fix missing intra-trun sample increment on seek

ends up reading wrong sample size/time

refs #25784

modules/demux/mp4/mp4.c

@@ -1841,6 +1841,7 @@ static void FragTrunSeekToTime( mp4_track_t *p_track, stime_t i_target_time )
 
             i_time += dur;
             i_pos += len;
+            i_sample++;
         }
     }