Add assert checking that window has no event handlers when destroyed.

Add assert checking that window has no event handlers when destroyed.

Forgetting to pop custom event handlers pushed onto the window before it is
destroyed will result in a crash later on, so try to detect this immediately
with an assert.

We might want to forcefully pop them off too, this could result in memory
leaks but would be still better than crashing.

Closes #11533.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63065 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775