Fix memory corruption in heightForWidth cache

Overflowing q_firstCachedHfw caused memory to be overwritten
before q_cachedHfws.

Change-Id: Ibbcc72380f426550cc0569a05c54cd1acd878b33
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

src/widgets/kernel/qlayoutitem.cpp

@@ -839,7 +839,7 @@ int QWidgetItemV2::heightForWidth(int width) const
         const QSize &size = q_cachedHfws[offset % HfwCacheMaxSize];
         if (size.width() == width) {
             if (q_hfwCacheSize == HfwCacheMaxSize)
-                q_firstCachedHfw = offset;
+                q_firstCachedHfw = offset % HfwCacheMaxSize;
             return size.height();
         }
     }