winmm GetDevCaps better pointer check patch

[Eric Pouech]

> I can see the evils of using IsBadWritePtr now but it seems that Microsoft
> does use it in their winmm implementation and to be compatible, wine 
> probably
> should too.  I think the patch is valid.
I'd rather think unless we find an app which absolutely requires it, 
we'd better leave it as it is (and don't make use of IsBadReadPtr).

A+