avformat/vividas: Check buffer size before allocation

Fixes: out of array access
Fixes: 15365/clusterfuzz-testcase-minimized-ffmpeg_DEMUXER_fuzzer-5716153105645568

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

@@ -251,6 +251,9 @@ static uint8_t *read_sb_block(AVIOContext *src, unsigned *size,
         *key = tmpkey;
     }
 
+    if (n < 8)
+        return NULL;
+
     buf = av_malloc(n);
     if (!buf)
         return NULL;