avformat/mov: Only fail for STCO/STSC contradictions if both exist

Fixes regression with playback of GF9720Repeal20the20Eighth20with20Helen20Linehan.m4a
See: crbug 822666
Found-by: "Mattias Wadman <mattias.wadman@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -4148,7 +4148,7 @@ static int mov_read_trak(MOVContext *c, AVIOContext *pb, MOVAtom atom)
                st->index);
         return 0;
     }
-    if (sc->stsc_count && sc->stsc_data[ sc->stsc_count - 1 ].first > sc->chunk_count) {
+    if (sc->chunk_count && sc->stsc_count && sc->stsc_data[ sc->stsc_count - 1 ].first > sc->chunk_count) {
         av_log(c->fc, AV_LOG_ERROR, "stream %d, contradictionary STSC and STCO\n",
                st->index);
         return AVERROR_INVALIDDATA;