xmlInitGlobals() is not thread-safe

If xmlInitGlobals() is called from two different threads before xmlThrDefMutex is initialized, a race could occur where a pthread_mutex_t created by xmlNewMutex() was leaked, and (worse) one thread might use the wrong (leaked) mutex, thus causing thread safety issues.

The fix is to add proper locking to make sure xmlThrDefMutex is initialized exactly once.