Cocoa: QNSView - fix memory leak of QNSViewMenuHelper

QNSViewMenuHelper is leaked, as dealloc did not nil it to decrement the
reference count.

Pick-to: 6.5 6.8 6.9 6.10
Fixes: QTBUG-131655
Fixes: QTBUG-137161
Change-Id: I783c400d1300046118ad3e12816e84709cc9d793
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qnsview.mm

@@ -168,6 +168,8 @@ QT_NAMESPACE_ALIAS_OBJC_CLASS(QNSViewMenuHelper);
 {
     qCDebug(lcQpaWindow) << "Deallocating" << self;
 
+    self.menuHelper = nil;
+
     [[NSNotificationCenter defaultCenter] removeObserver:self];
     [m_mouseMoveHelper release];