avformat/mvdec: Check stream numbers

Fixes: null pointer dereference
Fixes: 20768/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5638648978735104.fuzz

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 618a9bea65)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mvdec.c

@@ -361,6 +361,12 @@ static int mv_read_header(AVFormatContext *avctx)
         if ((ret = read_table(avctx, NULL, parse_global_var)) < 0)
             return ret;
 
+        if (mv->nb_audio_tracks < 0  || mv->nb_video_tracks < 0 ||
+           (mv->nb_audio_tracks == 0 && mv->nb_video_tracks == 0)) {
+            av_log(avctx, AV_LOG_ERROR, "Stream count is invalid.\n");
+            return AVERROR_INVALIDDATA;
+        }
+
         if (mv->nb_audio_tracks > 1) {
             avpriv_request_sample(avctx, "Multiple audio streams support");
             return AVERROR_PATCHWELCOME;