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
