libgsmdec: fix decoding of non 8khz gsm files

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/libgsm.c

@@ -156,7 +156,8 @@ static av_cold int libgsm_decode_init(AVCodecContext *avctx) {
 
     avctx->channels       = 1;
     avctx->channel_layout = AV_CH_LAYOUT_MONO;
-    avctx->sample_rate    = 8000;
+    if (!avctx->sample_rate)
+        avctx->sample_rate = 8000;
     avctx->sample_fmt     = AV_SAMPLE_FMT_S16;
 
     s->state = gsm_create();