lavc: Fix msvc compilation of utils.c with -WX (warnings treated as errors)

This fixes https://bugzilla.libav.org/show_bug.cgi?id=514.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/utils.c

@@ -762,6 +762,7 @@ void avcodec_default_release_buffer(AVCodecContext *s, AVFrame *pic)
 int avcodec_default_reget_buffer(AVCodecContext *s, AVFrame *pic)
 {
     av_assert0(0);
+    return AVERROR_BUG;
 }
 #endif