Also fixes 16314: XML: extractvalue() crash if vertical bar
xml.result, xml.test:
  Adding test case
item_xmlfunc.cc:
  Using root element as a context node,
  instead of NULL, with relative paths.


sql/item_xmlfunc.cc:
  Bug#16234 XML: Crash if ExtractValue()
  Also fixes 16314: XML: extractvalue() crash if vertical bar
  Use root element as a context node,
  instead of NULL, with relative paths.
mysql-test/t/xml.test:
  Adding test case
mysql-test/r/xml.result:
  Adding test case