Having had some trouble trying to use xmlListSort, I took a look at the
implementation.  It seems to work by copying the original list, clearing
the original list, and then inserting each element from the copy into
the original.  Apart from inefficiency, this has one major bug: clearing
the original list calls the user-supplied deallocator on each element,
making those elements unusable in the copy.  This resulted in accesses
to freed memory, easily found via valgrind.

- Josh Triplett