Simplify check for whether wxSizerItem window is null

No real changes, just write the check more compactly and without
redundant IsWindow() check.

src/common/sizer.cpp

@@ -878,8 +878,8 @@ bool wxSizer::Replace( size_t old, wxSizerItem *newitem )
     wxSizerItem *item = node->GetData();
     node->SetData(newitem);
 
-    if (item->IsWindow() && item->GetWindow())
-        item->GetWindow()->SetContainingSizer(NULL);
+    if (wxWindow* const w = item->GetWindow())
+        w->SetContainingSizer(NULL);
 
     delete item;