Don't send wxEVT_COMMAND_TREE_ITEM_MENU event without valid item in wxMSW.

Don't send wxEVT_COMMAND_TREE_ITEM_MENU event without valid item in wxMSW.

The generic wxTreeCtrl version only sends this event when the mouse is right
clicked on a valid item so do the same in wxMSW version too for consistency.

This is also consistent with wxEVT_COMMAND_TREE_ITEM_RIGHT_CLICK event and
avoids strange problems with unexpected wxEVT_COMMAND_TREE_ITEM_MENU events
generated on right double click.

Finally, replace the checks for item validity in the event handler in the
sample with asserts which should loudly complain if it happens to be invalid.

Closes #11226.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65903 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775