diff -uNr ffmpeg-0.4.9-pre1-org/libavcodec/utils.c ffmpeg-0.4.9-pre1/libavcodec/utils.c
--- ffmpeg-0.4.9-pre1-org/libavcodec/utils.c	2006-03-02 17:52:33.000000000 +0900
+++ ffmpeg-0.4.9-pre1/libavcodec/utils.c	2006-03-02 18:40:12.000000000 +0900
@@ -520,6 +520,10 @@
 {
     int ret;
 
+    if (buf_size <= 0) {
+        return 0;
+    }
+
     *frame_size_ptr= 0;
     ret = avctx->codec->decode(avctx, samples, frame_size_ptr, 
                                buf, buf_size);