a64: check that extradata exists before reading from it

libavformat/a64.c

@@ -35,6 +35,12 @@ static int a64_write_header(AVFormatContext *s)
         0x00, //charset_lifetime (multi only)
         0x00  //fps in 50/fps;
     };
+
+    if (avctx->extradata_size < 4) {
+        av_log(s, AV_LOG_ERROR, "Missing extradata\n");
+        return AVERROR(EINVAL);
+    }
+
     switch (avctx->codec->id) {
     case AV_CODEC_ID_A64_MULTI:
         header[2] = 0x00;