Replace use of C-style cast with something that is not a cast.

Avoids a warning in user code. We should add -Wold-style-cast to our
header checks.

Task-number: QTBUG-32735
Change-Id: Ic284e3a5df56789d51484364b34a9ebc026bb20a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>

src/xmlpatterns/api/qabstractxmlnodemodel.h

@@ -92,10 +92,10 @@ namespace QPatternist
         };
         void *pointer() const
         {
-            /* Constructing to qptrdiff means we avoid the warning "cast to pointer
-             * from integer of different size."
+            /* Constructing to qptrdiff means we avoid warnings.
              */
-            return (void *)qptrdiff(data);
+            char *null = 0;
+            return null + qptrdiff(data);
         }
 
         Data additionalData;