While xmlCleanupParser() should not be used unless complete control
is insured over the programe making sure libxml2 is not in use anywhere
It should still be usable, and allow a sequence of
    xmlInitParser();
    xmlCleanupParser();
calls if needed, the problem is that the thread key wasn't reallocated
on subsequent xmlinitParser() calls leading to corruption of pthread
keys used by the program.

* threads.c: make sure xmlCleanupParser() reset the pthread_once()
             global variable driving thread key allocation.