gh-99845: _PyObject_DictPointer(): fix dictoffset cast (GH-99922)

Cast size_t to Py_ssize_t, rather than casting it to long. On 64-bit
Windows, long is 32-bit whereas Py_ssize_t is 64-bit.
(cherry picked from commit 9707bf228e008485a3fbb63aa7ee28cf88014f91)

Co-authored-by: Victor Stinner <vstinner@python.org>