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The pointer left/entered window logic calls int_get_pointer_info which can return a window (and/or icon) handle of -1 if the pointer is over no window, such as might occur if the window has been deleted. The check for a duff window handle (in R3) was unfortunately made after it'd been dereferenced. Move this check up beforehand.

Fixes issue reported on Usenet https://groups.google.com/forum/#!topic/comp.sys.acorn.programmer/iR0_TNekiKg