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

@@ -226,6 +226,9 @@ class Item_nodeset_func :public Item_str_func
   {
     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"; }
 };