[xml] xmlCleanupGlobals reentrancy

["Rob Richards" <rrichards ctindustries net>]

I see that a test for xmlThrDefMutex == NULL was added in xmlCleanupGlobals
after the 2.5.8 release.

Is it also possible to also set xmlThrDefMutex = NULL after calling
xmlFreeMutex which would make the xmlCleanupParser reentrant safe again? I
see that there is more threading work going to be added, so figure I would
ask.

Thanks,

Rob Richards
rrichards ctindustries net