user32: Fix side effect in SubtractRect()

[Javier Cantero]

On Sun, Mar 23, 2014 at 12:18:04AM +0400, Nikolay Sivov wrote:
> That means we don't have any tests for it yet, so feel free to add
> test_SubtractRect().

Maybe a better idea is to do two separate patches:
 - First one with some basic conformance test cases for SubtractRect() (create
   the skeleton and verify that the test is well written and working)
 - Then (after the merge of the previous one) another with the fix and the test
   case that covers the fix added to the tests

I can send the first patch tomorrow if there is no objection.