ntdll: remove an unnecessary NULL check : Coverity

[Jeff L]

Jeff L wrote:
> Dmitry Timoshkov wrote:
>> Yes, 'attr' is used without a NULL check earlier, but that's an actual
>> bug, and that's where the code needs to be fixed.
>>
> So in this case my change is ok for the tidy up that it is, it just 
> that there is another problem.  Is this in bugzilla?
>
> Jeff
>
Looking a bit further it seems that NtCreateMailslotFile is only called 
from CreateMailslotW.  If there is a problem with a NULL then it stems 
from there. <http://source.winehq.org/ident?i=CreateMailslotW>

Jeff