avformat/4xm: Check that a video stream was created before returning packets for it

Fixes: assertion failure
Fixes: 23434/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5227750851084288.fuzz

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/4xm.c

@@ -218,6 +218,7 @@ static int fourxm_read_header(AVFormatContext *s)
     fourxm->track_count = 0;
     fourxm->tracks      = NULL;
     fourxm->fps         = (AVRational){1,1};
+    fourxm->video_stream_index = -1;
 
     /* skip the first 3 32-bit numbers */
     avio_skip(pb, 12);
@@ -326,6 +327,8 @@ static int fourxm_read_packet(AVFormatContext *s,
              * and size */
             if (size > INT_MAX - AV_INPUT_BUFFER_PADDING_SIZE - 8)
                 return AVERROR_INVALIDDATA;
+            if (fourxm->video_stream_index < 0)
+                return AVERROR_INVALIDDATA;
             if ((ret = av_new_packet(pkt, size + 8)) < 0)
                 return ret;
             pkt->stream_index = fourxm->video_stream_index;