If the menu item is merged then we should not release it

When a menu item is merged then we do not want to release it as it will
cause a problem with the Application menu. Therefore we just hide it
instead.

Change-Id: Ia9f7acfe063001111769841c9a360cac369f7ed7
Reviewed-by: James Turner <james.turner@kdab.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>

src/plugins/platforms/cocoa/qcocoamenuitem.mm

@@ -104,9 +104,9 @@ QCocoaMenuItem::~QCocoaMenuItem()
 {
     if (m_merged) {
         [m_native setHidden:YES];
+    } else {
+        [m_native release];
     }
-
-    [m_native release];
 }
 
 void QCocoaMenuItem::setText(const QString &text)