user32: Fix side effect in SubtractRect()

[Nikolay Sivov]

On 3/23/2014 00:06, Javier Cantero wrote:
>> 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?
That means we don't have any tests for it yet, so feel free to add 
test_SubtractRect().
>