Add a NULL pointer check to avcodec_close() this should prevent a segfault

when closing without open.

Originally committed as revision 20495 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/utils.c

@@ -675,7 +675,7 @@ int avcodec_close(AVCodecContext *avctx)
 
     if (HAVE_THREADS && avctx->thread_opaque)
         avcodec_thread_free(avctx);
-    if (avctx->codec->close)
+    if (avctx->codec && avctx->codec->close)
         avctx->codec->close(avctx);
     avcodec_default_free_buffers(avctx);
     av_freep(&avctx->priv_data);