audio_get_buffer: add assert to ensure nb_extended_buf has a safe value

May silence CID991853
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -478,8 +478,10 @@ static int audio_get_buffer(AVCodecContext *avctx, AVFrame *frame)
             av_freep(&frame->extended_buf);
             return AVERROR(ENOMEM);
         }
-    } else
+    } else {
         frame->extended_data = frame->data;
+        av_assert0(frame->nb_extended_buf == 0);
+    }
 
     for (i = 0; i < FFMIN(planes, AV_NUM_DATA_POINTERS); i++) {
         frame->buf[i] = av_buffer_pool_get(pool->pools[0]);