Fix memory leak in XMLSERIALIZE(... INDENT).

Fix memory leak in XMLSERIALIZE(... INDENT).

xmltotext_with_options sometimes tries to replace the existing
root node of a libxml2 document.  In that case xmlDocSetRootElement
will unlink and return the old root node; if we fail to free it,
it's leaked for the remainder of the session.  The amount of memory
at stake is not large, a couple hundred bytes per occurrence, but
that could still become annoying in heavy usage.

Our only other xmlDocSetRootElement call is not at risk because
it's working on a just-created document, but let's modify that
code too to make it clear that it's dependent on that.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Discussion: https://postgr.es/m/1358967.1747858817@sss.pgh.pa.us
Backpatch-through: 16