Fix cp_binding_level reuse logic

gcc/cp/ChangeLog:

	* name-lookup.c (begin_scope): After reusing a cp_binding_level
	structure, update free_binding_level before the structure's
	level_chain field gets cleared, not after.

From-SVN: r232965

gcc/cp/ChangeLog

+2016-01-29  Patrick Palka  <ppalka@gcc.gnu.org>
+
+	* name-lookup.c (begin_scope): After reusing a cp_binding_level
+	structure, update free_binding_level before the structure's
+	level_chain field gets cleared, not after.
+
 2016-01-28  Jason Merrill  <jason@redhat.com>
 
 	PR c++/67407

gcc/cp/name-lookup.c

@@ -1557,8 +1557,8 @@ begin_scope (scope_kind kind, tree entity)
   if (!ENABLE_SCOPE_CHECKING && free_binding_level)
     {
       scope = free_binding_level;
-      memset (scope, 0, sizeof (cp_binding_level));
       free_binding_level = scope->level_chain;
+      memset (scope, 0, sizeof (cp_binding_level));
     }
   else
     scope = ggc_cleared_alloc<cp_binding_level> ();