Fix use-after-free bugs when calling xmlTextReaderClose() before...

Fix use-after-free bugs when calling xmlTextReaderClose() before xmlFreeTextReader() on post-validating parser

When creating an xmlTextReaderPtr using xmlReaderForMemory(),
there are two optional API functions that can be used:
- xmlTextReaderClose() may be called prior to calling
  xmlFreeTextReader() to free parsing resources and close the
  xmlTextReaderPtr without freeing it.
- xmlTextReaderCurrentDoc() may be called to return an
  xmlDocPtr that's owned by the caller, and must be free using
  xmlFreeDoc() after calling xmlFreeTextReader().

The use-after-free issues occur when calling
xmlTextReaderClose() before xmlFreeTextReader(), with different
issues occurring depending on whether xmlTextReaderCurrentDoc()
is also called.

* xmlreader.c:
(xmlFreeTextReader):
- Move code to xmlTextReaderClose(), remove duplicate code, and
  call xmlTextReaderClose() if it hasn't been called yet.
(xmlTextReaderClose):
- Move call to xmlFreeNode(reader->faketext) from
  xmlFreeTextReader() to fix a use-after-free bug when calling
  xmlTextReaderClose() before xmlFreeTextReader(), but not when
  using xmlTextReaderCurrentDoc().  The bug was introduced in
  2002 by commit beb70bd3.  In 2009 commit f4653dcd fixed the
  use-after-free that occurred every time xmlFreeTextReader()
  was called, but not the case where xmlTextReaderClose() was
  called first.
- Move post-parsing validation code from xmlFreeTextReader() to
  fix a second use-after-free when calling xmlTextReaderClose()
  before xmlFreeTextReader().  This regressed in v2.9.10 with
  commit 57a3af56.
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