Adding test.
item_xmlfunc.cc:
  Bug #18171 XML: ExtractValue: the XPath position() function crashes the server!
  Disallowing use of position() and last() without context.


sql/item_xmlfunc.cc:
  Bug #18171 XML: ExtractValue: the XPath position() function crashes the server!
  Disallowing use of position() and last() without context.
mysql-test/t/xml.test:
  Adding test.
mysql-test/r/xml.result:
  Adding test.