avformat/wtvdec: Check length of read mpeg2_descriptor

Fixes: Use of uninitialized value
Fixes: 70900/clusterfuzz-testcase-minimized-ffmpeg_dem_WTV_fuzzer-6286909377150976

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

libavformat/wtvdec.c

@@ -846,7 +846,8 @@ static int parse_chunks(AVFormatContext *s, int mode, int64_t seekts, int *len_p
                 }
 
                 buf_size = FFMIN(len - consumed, sizeof(buf));
-                avio_read(pb, buf, buf_size);
+                if (avio_read(pb, buf, buf_size) != buf_size)
+                    return AVERROR_INVALIDDATA;
                 consumed += buf_size;
                 ff_parse_mpeg2_descriptor(s, st, 0, &pbuf, buf + buf_size, NULL, 0, 0, NULL);
             }