gsmdec: fix decoding of non 8khz gsm files

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

libavcodec/gsmdec.c

@@ -37,7 +37,8 @@ static av_cold int gsm_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;
 
     switch (avctx->codec_id) {