Bug#12428404 MYSQLD.EXE CRASHES WHEN EXTRACTVALUE() IS CALLED

WITH MALFORMED XPATH EXP
Problem:
A malformed XPATH expression in the ExtractValue query is 
causing a server crash. This malformed XPATH expression is
resulted when the position attribute in the substring function
contains ".." in the beginning.
Solution:
The original crash is happening because the "../" is being 
evaluated prematurely. It tries to access XML while it 
hasn't been parsed yet. The premature evaluation is happening
because the val_nodeset function is being set to constant, 
in which case we proceed to evaluate them in JOIN:prepare
stage only. The solution to this is setting the val_nodeset
functions as non-constant. This forces us to evaluate the function
 in the JOIN:exec stage and thus avoid any premature evaluation of
the XML strings.

sql/item_xmlfunc.cc

@@ -220,6 +220,9 @@ public:
   {
     max_length= MAX_BLOB_WIDTH;
     collation.collation= pxml->charset();
+    // To avoid premature evaluation, mark all nodeset functions as non-const.
+    used_tables_cache= RAND_TABLE_BIT;
+    const_item_cache= false;
   }
   const char *func_name() const { return "nodeset"; }
 };