Do not call xmlCleanupParser from liblangtag

[stbergmann]

Despite its name, it is a cleanup function for libxml2 that must only be called
when the process as a whole no longer needs libxml2. Calling it from a library
like liblangtag is not appropriate (just like liblangtag does not call
xmlInitParser, either).

In LibreOffice, this caused confusion about pthread keys for thread-local
storage (xmlCleanupParser calls pthread_key_delete, but apparently if libxml2 is
later used by unrelated code in LibreOffice again, it still uses the stale key,
and if that key is now reused via an unrelated pthread_key_create, arbitrary
crashes happen during thread termination in pthread's __nptl_deallocate_tsd),
cf. <http://cgit.freedesktop.org/libreoffice/core/commit/
?id=5e6c595e9d698caf23ae68d38b9cab30ff9c9b94> "Do not call xmlCleanupParser from
liblangtag."

[tagoh]

Aha. but I see there are some memory leaks without it. I'm not quite sure if there are any way to clean it up.

[stbergmann]

"I'm not quite sure if there are any way to clean it up.": I fear there is no way for the library to clean it up, that rather the design of libxml2 requires that once it is used in a program, and may potentially be used again, then its fundamental (not per-document) resources can only be reclaimed at program exit. See e.g. the discussion at https://bugzilla.redhat.com/show_bug.cgi?id=554903" Please print a warning (or abort) if xmlCleanupParser() is called twice."

[tagoh]

Okay. thanks for explanation and patch!