avcodec/wavpackenc: Headers are per channel

Fixes: 1b8b83a53bfa751f01b1daa65a4758db/signal_sigabrt_7ffff6ae7cb7_7488_403f71d1a2565b598d01b6cb110fac8f.aiff
Fixes: assertion failure

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/wavpackenc.c

@@ -2878,7 +2878,7 @@ static int wavpack_encode_frame(AVCodecContext *avctx, AVPacket *avpkt,
     }
 
     buf_size = s->block_samples * avctx->channels * 8
-             + 200 /* for headers */;
+             + 200 * avctx->channels /* for headers */;
     if ((ret = ff_alloc_packet2(avctx, avpkt, buf_size, 0)) < 0)
         return ret;
     buf = avpkt->data;