avformat/thp: Require a video stream

The demuxer code assumes the existence of a video stream

Fixes: assertion failure
Fixes: 21512/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5699660783288320

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 97c78caf3e)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/thp.c

@@ -145,6 +145,9 @@ static int thp_read_header(AVFormatContext *s)
         }
     }
 
+    if (!thp->vst)
+        return AVERROR_INVALIDDATA;
+
     return 0;
 }