msvcirt: Avoid "array subscript above array bounds" warnings in test_ostrstream and test_strstreambuf.

[Filip Frackiewicz]

Hi Gerald,

In wine-1.9.22-129-g906e770 I see the array subscript warnings in the
following location:

../../../../wine/dlls/msvcirt/tests/msvcirt.c: In function ‘test_strstream’:
../../../../wine/dlls/msvcirt/tests/msvcirt.c:6861:34: warning: array
subscript is above array bounds [-Warray-bounds]
     ok(pssb->base.ebuf == buffer + 0x7fffffff || pssb->base.ebuf == (char*) -1,
                           ~~~~~~~^~~~~~~~~~~~
../../../../wine/dlls/msvcirt/tests/msvcirt.c:6868:35: warning: array
subscript is above array bounds [-Warray-bounds]
     ok(pssb->base.epptr == buffer + 0x7fffffff || pssb->base.epptr ==
(char*) -1,

Thought I'd let you know as I just encountered this while compiling
the 64-bit version of Wine.

Cheers,

Filip