wavpackenc: proper buffer allocation

The allocation didn't account for headers, that can be easily 79 bytes.
As a result, buffers allocated for a few samples (e.g. 5 in the original
bug) could be undersized.

Fixed ticket #2881.

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/wavpackenc.c

@@ -2876,10 +2876,11 @@ static int wavpack_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
             return AVERROR(ENOMEM);
     }
 
-    if ((ret = ff_alloc_packet2(avctx, avpkt, s->block_samples * avctx->channels * 8)) < 0)
+    buf_size = s->block_samples * avctx->channels * 8
+             + 200 /* for headers */;
+    if ((ret = ff_alloc_packet2(avctx, avpkt, buf_size)) < 0)
         return ret;
     buf = avpkt->data;
-    buf_size = avpkt->size;
 
     for (s->ch_offset = 0; s->ch_offset < avctx->channels;) {
         set_samplerate(s);