[AD] Graphical corruption with bogus set_mouse_sprite_focus parameters.

[Grzegorz Adam Hankiewicz <gradha@xxxxxxxxxx>]

I found out that calling set_mouse_sprite_focus(-50, -50) in exgui.c
generated noticeable graphical corruption. Even if this is a bug in
the mouse sprite update code, it doesn't seem logical to be able
to set the focus of the sprite with coordinates outside of it's
range. That's just asking for troubles. Proposed patch attached.

Attachment: set_mouse_sprite_focus.diff.gz
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