gsmdec: always set channel layout and sample rate at initialization

Only mono 8kHz is supported.

libavcodec/gsmdec.c

@@ -24,6 +24,7 @@
  * GSM decoder
  */
 
+#include "libavutil/audioconvert.h"
 #include "avcodec.h"
 #include "get_bits.h"
 #include "msgsmdec.h"
@@ -35,7 +36,7 @@ static av_cold int gsm_init(AVCodecContext *avctx)
     GSMContext *s = avctx->priv_data;
 
     avctx->channels       = 1;
-    if (!avctx->sample_rate)
+    avctx->channel_layout = AV_CH_LAYOUT_MONO;
     avctx->sample_rate    = 8000;
     avctx->sample_fmt     = AV_SAMPLE_FMT_S16;