BUG#18037: Fix stack corruption in THD::rollback_item_tree_changes().

Stored procedure execution sometimes placed the address of auto variables
in the list of Item changes to undo in THD::rollback_item_tree_changes().
This could cause stack corruption.


sql/sp_head.cc:
  Avoid storing address of auto variables in global rollback list, to
  prevent stack memory corruption.
sql/sp_head.h:
  Avoid storing address of auto variables in global rollback list, to
  prevent stack memory corruption.
sql/sp_rcontext.cc:
  Avoid storing address of auto variables in global rollback list, to
  prevent stack memory corruption.
sql/sp_rcontext.h:
  Avoid storing address of auto variables in global rollback list, to
  prevent stack memory corruption.
sql/sql_class.cc:
  Avoid storing address of auto variables in global rollback list, to
  prevent stack memory corruption.

sql/sql_class.cc

@@ -1877,7 +1877,7 @@ bool select_dumpvar::send_data(List<Item> &items)
       if ((yy=var_li++)) 
       {
 	if (thd->spcont->set_variable(current_thd, yy->get_var_idx(),
-                                      *it.ref()))
+                                      it.ref()))
 	  DBUG_RETURN(1);
       }
     }