user32: Fix side effect in SubtractRect()

[Javier Cantero]

> Hi, Jose.
>
> Please add a test case for this change to user32/tests/uitools.c
>

OK, but... I don't see any test function for SubtractRect() in
dlls/user32/tests/uitools.c (in fact there is only 1 test case in that
file).

Am I missing something?