Cocoa integration - avoid dangling menuitem pointer

Since QCocoaMenu can live longer than its m_attachedItem pointer,
this pointer is becoming invalid after QCocoaMenuItem deleted
(and its 'm_native' was released).

Task-number: QTBUG-53251
Change-Id: I6d97b75b2c09e2443cd21415c5db94206d5d89ce
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@theqtcompany.com>
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>