avformat/vividas: Use equals check with n in read_sb_block()

Fixes: OOM
Fixes: 27780/clusterfuzz-testcase-minimized-ffmpeg_dem_VIVIDAS_fuzzer-5097985075314688

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

libavformat/vividas.c

@@ -267,7 +267,7 @@ static uint8_t *read_sb_block(AVIOContext *src, unsigned *size,
     *size = n;
     n -= 8;
 
-    if (avio_read(src, buf+8, n) < n) {
+    if (avio_read(src, buf+8, n) != n) {
         av_free(buf);
         return NULL;
     }