wmaenc: require a large enough output buffer to prevent overwrites

The maximum theoretical frame size is around 17000 bytes. Although in
practice it will generally be much smaller, we require a larger buffer
just to be safe.

CC: libav-stable@libav.org

libavcodec/wmaenc.c

@@ -365,6 +365,11 @@ static int encode_superframe(AVCodecContext *avctx,
         }
     }
 
+    if (buf_size < 2 * MAX_CODED_SUPERFRAME_SIZE) {
+        av_log(avctx, AV_LOG_ERROR, "output buffer size is too small\n");
+        return AVERROR(EINVAL);
+    }
+
 #if 1
     total_gain= 128;
     for(i=64; i; i>>=1){