Also fix other Ring ctor

...after 8b12e63b "Don't downcast from Ring<T>
base class ctor/dtor to derived class T"

Change-Id: I734841a78e272e985d784bd756412e1af90b2df5

sw/inc/ring.hxx

@@ -127,8 +127,7 @@ namespace sw
     {
         if( pObj )
         {
-            value_type* pThis = static_cast< value_type* >(this);
-            algo::link_before(pObj, pThis);
+            algo::link_before(pObj, this);
         }
     }