Fix: No sound is exported with ffmpeg 4.4

We were not assigning the amount of sound channels to the output frames.
Newer ffmpeg releases has sanity checks in place and doesn't fall back
to two channels anymore.

source/blender/blenkernel/intern/writeffmpeg.c

@@ -164,6 +164,7 @@ static int write_audio_frame(FFMpegContext *context)
   frame->pts = context->audio_time / av_q2d(c->time_base);
   frame->nb_samples = context->audio_input_samples;
   frame->format = c->sample_fmt;
+  frame->channels = c->channels;
 #      ifdef FFMPEG_HAVE_FRAME_CHANNEL_LAYOUT
   frame->channel_layout = c->channel_layout;
 #      endif