libavcodec/dcadec: adjust the `ch_layout` when downmix is active

Applications making use of this codec with the `downmix` option are
segfaulting unless the `ch_layout` is overridden after `avcodec_open2`
as can be seen in projects like MythTV[1]

This patch fixes this by overriding the ch_layout as done in other
decoders such as AC3.

1: af6f362a14/mythtv/libs/libmythtv/decoders/avformatdecoder.cpp (L4607)

Signed-off-by: Geoffrey McRae <geoff@hostfission.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/dcadec.c

@@ -366,12 +366,19 @@ FF_ENABLE_DEPRECATION_WARNINGS
 
     if (s->downmix_layout.nb_channels) {
         if (!av_channel_layout_compare(&s->downmix_layout, &(AVChannelLayout)AV_CHANNEL_LAYOUT_STEREO) ||
-            !av_channel_layout_compare(&s->downmix_layout, &(AVChannelLayout)AV_CHANNEL_LAYOUT_STEREO_DOWNMIX))
+            !av_channel_layout_compare(&s->downmix_layout, &(AVChannelLayout)AV_CHANNEL_LAYOUT_STEREO_DOWNMIX)) {
             s->request_channel_layout = DCA_SPEAKER_LAYOUT_STEREO;
-        else if (!av_channel_layout_compare(&s->downmix_layout, &(AVChannelLayout)AV_CHANNEL_LAYOUT_5POINT0))
+            av_channel_layout_uninit(&avctx->ch_layout);
+            avctx->ch_layout = (AVChannelLayout)AV_CHANNEL_LAYOUT_STEREO;
+        } else if (!av_channel_layout_compare(&s->downmix_layout, &(AVChannelLayout)AV_CHANNEL_LAYOUT_5POINT0)) {
             s->request_channel_layout = DCA_SPEAKER_LAYOUT_5POINT0;
-        else if (!av_channel_layout_compare(&s->downmix_layout, &(AVChannelLayout)AV_CHANNEL_LAYOUT_5POINT1))
+            av_channel_layout_uninit(&avctx->ch_layout);
+            avctx->ch_layout = (AVChannelLayout)AV_CHANNEL_LAYOUT_5POINT0;
+        } else if (!av_channel_layout_compare(&s->downmix_layout, &(AVChannelLayout)AV_CHANNEL_LAYOUT_5POINT1)) {
             s->request_channel_layout = DCA_SPEAKER_LAYOUT_5POINT1;
+            av_channel_layout_uninit(&avctx->ch_layout);
+            avctx->ch_layout = (AVChannelLayout)AV_CHANNEL_LAYOUT_5POINT1;
+        }
         else
             av_log(avctx, AV_LOG_WARNING, "Invalid downmix layout\n");
     }