Back-patch changes of 2009-05-13 in xml.c's memory management.

Back-patch changes of 2009-05-13 in xml.c's memory management.

I was afraid to do this when these changes were first made, but now that
8.4 has seen some field use it should be all right to back-patch.  These
changes are really quite necessary in order to give xml.c any hope of
co-existing with loadable modules that also wish to use libxml2.