avcodec: validate the channel layout vs. channel count for decoders

Set avctx->channel_layout to 0 if the channel count does not match
avctx->channels.

libavcodec/utils.c

@@ -810,6 +810,15 @@ int attribute_align_arg avcodec_open2(AVCodecContext *avctx, AVCodec *codec, AVD
             goto free_and_end;
         }
     }
+
+    if (av_codec_is_decoder(avctx->codec)) {
+        /* validate channel layout from the decoder */
+        if (avctx->channel_layout &&
+            av_get_channel_layout_nb_channels(avctx->channel_layout) != avctx->channels) {
+            av_log(avctx, AV_LOG_WARNING, "channel layout does not match number of channels\n");
+            avctx->channel_layout = 0;
+        }
+    }
 end:
     entangled_thread_counter--;