amrnbdec: allow sample rates other than 8khz

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

libavcodec/amrnbdec.c

@@ -169,6 +169,7 @@ static av_cold int amrnb_decode_init(AVCodecContext *avctx)
 
     avctx->channels       = 1;
     avctx->channel_layout = AV_CH_LAYOUT_MONO;
+    if (!avctx->sample_rate)
         avctx->sample_rate = 8000;
     avctx->sample_fmt     = AV_SAMPLE_FMT_FLT;