prevent multiple av_register_all()

Originally committed as revision 3570 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/allformats.c

@@ -26,6 +26,12 @@
  */
 void av_register_all(void)
 {
+    static int inited = 0;
+    
+    if (inited != 0)
+        return;
+    inited = 1;
+
     avcodec_init();
     avcodec_register_all();