macOS: wrong GlobalRef deleted in GlassMenu

When `com.sun.glass.ui.mac.MacMenuDelegate` is invoked with an empty callback, there is a possibility that later a globalRef will be deleted for wrong reasons.

The problem is in the native implementation in GlassMenu.m. The invoked `Java_com_sun_glass_ui_mac_MacMenuDelegate__1setCallback` will delete the JNI globalRef to the current `menu->jCallback`, but it does not remove the value of `menu->jCallback` when the new callback is `null`.
When later the `menu` object is released, the `dealloc` method will check if the `menu->jCallback` is null, and if not, it will invoke `env->DeleteGlobalRef` on it.

There is a possibility that in the meantime (between invoking setCallback with an empty callback and deleting the menu) the old value is reused as the result of a next call to env->newGlobalRef, in which case that new globalRef will suddenly become deleted.

This caused a crash in applications where the first globalRef was reused in a submitForLaterInvocation call.

Solution:
set the value of `menu->jCallback` to `null` immediately after invoking `(*env)->DeleteGlobalRef(env, menu->jCallback);` in `Java_com_sun_glass_ui_mac_MacMenuDelegate__1setCallback`