Don't assume that WM_DESTROY is the last message to be received by a

Don't assume that WM_DESTROY is the last message to be received by a
window.  Since the child windows are still alive and kicking by the
time the parent receives WM_DESTROY, it's still possible to receive
other messages after WM_DESTROY.

If we free any window specific data when handling WM_DESTROY, we
should reset the window data field as well, and check if we have a
valid pointer when retrieving the window data field later.

ticket: 5584

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@19627 dc483132-0cff-0310-8789-dd5450dbe970