MDEV-15118 ExtractValue(xml,something_complex) does not work

Item_xml_str_func::fix_fields() used a local "String tmp" as a buffer
for args[1]->val_str(). "tmp" was freed at the end of fix_fields(),
while Items created during my_xpath_parse() still pointed to its fragments.

Adding a new member Item_xml_str_func::m_xpath_query and store the result
of args[1]->val_str() into it.

sql/item_xmlfunc.h

@@ -67,6 +67,7 @@ protected:
       return parse(res, cache);
     }
   };
+  String m_xpath_query; // XPath query text
   Item *nodeset_func;
   XML xml;
   bool get_xml(XML *xml, bool cache= false)