ntdll: Fix RtlIntegerToUnicodeString so it won't overflow

[Dan Kegel]

James wrote:
> > It copies str->Length + sizeof(WCHAR) to the destination buffer
> > according to james' testcases.
>
> No, the length is indeterminate.

Oh, is that the test that checks whether not double-null terminating
works?  I think that test needs rewriting to be determinate...
e.g. by putting in a list that has known bad (but properly terminated)
elements after the nominal end, and making sure the bad part
doesn't get processed... or something...