avformat/mov: validate chunk_count vs stsc_data

Bad content may contain stsc boxes with a first_chunk index that
exceeds stco.entries (chunk_count). This ammends the existing check to
include cases where chunk_count == 0. It also patches up the case
when stsc refers to unknown chunks, but stts has no samples (so we
can simply ignore stsc).
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -2697,8 +2697,11 @@ static inline int64_t mov_get_stsc_samples(MOVStreamContext *sc, unsigned int in
 
     if (mov_stsc_index_valid(index, sc->stsc_count))
         chunk_count = sc->stsc_data[index + 1].first - sc->stsc_data[index].first;
-    else
+    else {
+        // Validation for stsc / stco  happens earlier in mov_read_stsc + mov_read_trak.
+        av_assert0(sc->stsc_data[index].first <= sc->chunk_count);
         chunk_count = sc->chunk_count - (sc->stsc_data[index].first - 1);
+    }
 
     return sc->stsc_data[index].count * (int64_t)chunk_count;
 }
@@ -4170,6 +4173,13 @@ static int mov_read_trak(MOVContext *c, AVIOContext *pb, MOVAtom atom)
 
     c->trak_index = -1;
 
+    // Here stsc refers to a chunk not described in stco. This is technically invalid,
+    // but we can overlook it (clearing stsc) whenever stts_count == 0 (indicating no samples).
+    if (!sc->chunk_count && !sc->stts_count && sc->stsc_count) {
+        sc->stsc_count = 0;
+        av_freep(&sc->stsc_data);
+    }
+
     /* sanity checks */
     if ((sc->chunk_count && (!sc->stts_count || !sc->stsc_count ||
                             (!sc->sample_size && !sc->sample_count))) ||
@@ -4178,7 +4188,7 @@ static int mov_read_trak(MOVContext *c, AVIOContext *pb, MOVAtom atom)
                st->index);
         return 0;
     }
-    if (sc->chunk_count && sc->stsc_count && sc->stsc_data[ sc->stsc_count - 1 ].first > sc->chunk_count) {
+    if (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;