avcodec/mpegaudiodec: Reset dither and mdct state

This makes the mp3 decoder produce the same result when repeatly flushing and decoding
Suggested-by: Dale Curtis <dalecurtis@chromium.org>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/mpegaudiodec_template.c

@@ -1705,7 +1705,9 @@ static int decode_frame(AVCodecContext * avctx, void *data, int *got_frame_ptr,
 static void mp_flush(MPADecodeContext *ctx)
 {
     memset(ctx->synth_buf, 0, sizeof(ctx->synth_buf));
+    memset(ctx->mdct_buf, 0, sizeof(ctx->mdct_buf));
     ctx->last_buf_size = 0;
+    ctx->dither_state = 0;
 }
 
 static void flush(AVCodecContext *avctx)