Make a plausibility check when setting channel layout for TrueHD.

TrueHD supports more channels than FFmpeg, so a valid sample
could set the channel layout to a value that represents less
channels than the sample actually consists of.

libavcodec/mlp_parser.c

@@ -325,6 +325,10 @@ static int mlp_parse(AVCodecParserContext *s,
                 avctx->channels = truehd_channels(mh.channels_thd_stream1);
                 avctx->channel_layout = truehd_layout(mh.channels_thd_stream1);
             }
+            if (av_get_channel_layout_nb_channels(avctx->channel_layout) != avctx->channels) {
+                avctx->channel_layout = 0;
+                av_log_ask_for_sample(avctx, "Unknown channel layout.");
+            }
         }
 
         if (!mh.is_vbr) /* Stream is CBR */