Fix crashes in 64 bit Mozilla and Chrome-based applications.
Gecko vbuf backend: Don't pass a pointer to a 32 bit int to IAccessible2::get_windowHandle, which expects a pointer to a 64 bit HWND.
HWND is a 64 bit type on x64, but Windows guarantees that only 32 bits are used. However, just because the other bits aren't used doesn't mean they won't be written, thus potentially corrupting the stack.
Crash originally reported by @MarcoZehe in Mozilla bug 1311062.
