avformat/vividas: check length in read_vblock()

Fixes: out of array access
Fixes: 15166/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5731062396747776

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

libavformat/vividas.c

@@ -205,7 +205,7 @@ static uint8_t *read_vblock(AVIOContext *src, uint32_t *size,
     decode_block(tmp, tmp, 4, key, k2, align);
 
     n = get_v(tmp);
-    if (!n)
+    if (n < 4)
         return NULL;
 
     buf = av_malloc(n);