mov: validate time_scale

A negative timescale doesn't make sense and triggers assertions in
av_rescale_rnd.

Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavformat/mov.c

@@ -1223,6 +1223,10 @@ static int mov_read_mdhd(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     mov_metadata_creation_time(&st->metadata, creation_time);
 
     sc->time_scale = avio_rb32(pb);
+    if (sc->time_scale <= 0) {
+        av_log(c->fc, AV_LOG_ERROR, "Invalid mdhd time scale %d\n", sc->time_scale);
+        return AVERROR_INVALIDDATA;
+    }
     st->duration = (version == 1) ? avio_rb64(pb) : avio_rb32(pb); /* duration */
 
     lang = avio_rb16(pb); /* language */
@@ -1248,7 +1252,10 @@ static int mov_read_mvhd(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     }
     mov_metadata_creation_time(&c->fc->metadata, creation_time);
     c->time_scale = avio_rb32(pb); /* time scale */
-
+    if (c->time_scale <= 0) {
+        av_log(c->fc, AV_LOG_ERROR, "Invalid mvhd time scale %d\n", c->time_scale);
+        return AVERROR_INVALIDDATA;
+    }
     av_log(c->fc, AV_LOG_TRACE, "time scale = %i\n", c->time_scale);
 
     c->duration = (version == 1) ? avio_rb64(pb) : avio_rb32(pb); /* duration */