libopencore-amrdec: allow sampling rates other than 8khz

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

libavcodec/libopencore-amr.c

@@ -31,7 +31,8 @@ static void amr_decode_fix_avctx(AVCodecContext *avctx)
 {
     const int is_amr_wb = 1 + (avctx->codec_id == AV_CODEC_ID_AMR_WB);
 
-    avctx->sample_rate = 8000 * is_amr_wb;
+    if (!avctx->sample_rate)
+        avctx->sample_rate = 8000 * is_amr_wb;
 
     if (avctx->channels > 1) {
         av_log_missing_feature(avctx, "multi-channel AMR", 0);