avfilter/buffersrc: fix overriding unknown channel layouts with negotiated one

Fixes ffplay playback of unknown layouts, when SDL directly supports the audio
format, such as:

ffplay -f lavfi anullsrc=cl=2C,aformat=s16

Without the patch, "Channel layout change is not supported" errors are
generated because buffersrc (unknown 2 channel) and buffersink (stereo)
negotiated a stereo layout, but the stereo layout was never stored in the
BufferSourceContext.

This fixes a regression of 7251f90972, but this
is more of a regression of the avfilter channel layout conversion
(1f96db959c).

Signed-off-by: Marton Balint <cus@passwd.hu>
(cherry picked from commit 2df2b4067e)

libavfilter/buffersrc.c

@@ -446,7 +446,7 @@ static int config_props(AVFilterLink *link)
         }
         break;
     case AVMEDIA_TYPE_AUDIO:
-        if (!c->ch_layout.nb_channels) {
+        if (!c->ch_layout.nb_channels || c->ch_layout.order == AV_CHANNEL_ORDER_UNSPEC) {
             int ret = av_channel_layout_copy(&c->ch_layout, &link->ch_layout);
             if (ret < 0)
                 return ret;