dcadec: check layout & channel count for consistency.

Fixes out of array accesses

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dcadec.c

@@ -2249,6 +2249,11 @@ static int dca_decode_frame(AVCodecContext *avctx, void *data,
                 s->channel_order_tab[channels - 1 - !!s->lfe] < 0)
                 return AVERROR_INVALIDDATA;
 
+            if (av_get_channel_layout_nb_channels(avctx->channel_layout) != channels) {
+                av_log(avctx, AV_LOG_ERROR, "Number of channels %d mismatches layout %d\n", channels, av_get_channel_layout_nb_channels(avctx->channel_layout));
+                return AVERROR_INVALIDDATA;
+            }
+
             if (avctx->request_channels == 2 && s->prim_channels > 2) {
                 channels = 2;
                 s->output = DCA_STEREO;