avformat/allformats: Making av_register_all() thread-safe.

When multiple threads tries to call av_register_all(), the first thread sets
initialized to 1 and do the register process. At the same time, other thread might
also call av_register_all(), which returns immediately because initialized is set to 1
(even when it has not completed registering codecs). We can avoid this problem
if we set initialised to 1 while exiting from function.

Github: Closes #196

libavformat/allformats.c

@@ -47,7 +47,6 @@ void av_register_all(void)
 
     if (initialized)
         return;
-    initialized = 1;
 
     avcodec_register_all();
 
@@ -371,4 +370,6 @@ void av_register_all(void)
     REGISTER_DEMUXER (LIBGME,           libgme);
     REGISTER_DEMUXER (LIBMODPLUG,       libmodplug);
     REGISTER_MUXDEMUX(LIBNUT,           libnut);
+
+    initialized = 1;
 }