Fix QSystemTrayIcon stale pointer access

The child imageCell is a NSView and may be retained by Cocoa,
which means it may outlive the parent QNSStatusItem.

Clear its parent pointer to avoid referencing a stale
pointer.

Task-number: QTBUG-47929
Change-Id: I6078070b8c9f512ecd034fee4e54b1d8282dabdf
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/plugins/platforms/cocoa/qcocoasystemtrayicon.mm

@@ -372,6 +372,7 @@ QT_END_NAMESPACE
 - (void)dealloc {
     [[NSStatusBar systemStatusBar] removeStatusItem:item];
     [[NSNotificationCenter defaultCenter] removeObserver:imageCell];
+    imageCell.parent = nil;
     [imageCell release];
     [item release];
     [super dealloc];