XPath without a XPath syntax error
item_xmlfunc.cc:
  Error message didn't happen because after
  a failing attempt to parse RelativeLocationPath,
  my_xpath_parse_AbsoluteLocationPath() returned success.
  Changeing logic a bit:
  - Try to parse EOF first, return success if true.
  - Then try to parse RelativeLocationPath(), return success if true.
  - Otherwise return failure.
xml.result:
  Adding test case.
  Also, this change made it possible to generate 
  an error message earlier in the case of another
  bad XPATH syntax.
xml.test:
  Adding test case.


sql/item_xmlfunc.cc:
  Bug #18172 XML: Extractvalue() accepts mallformed XPath without a XPath syntax error
  Error message didn't happen because after
  a failing attempt to parse RelativeLocationPath(),
  my_xpath_parse_AbsoluteLocationPath() returned with 1.
  Changeing logic a bit: check for EOF first.
  Then try to parse RelativeLocationPath().
  If the latter fails, return failure.
mysql-test/t/xml.test:
  Adding test case.
mysql-test/r/xml.result:
  Adding test case.
  Also, this change makes error message
  to be generated earlier in the case of another
  bad XPATH syntax.