unsafe.Sizeof(windows.SECURITY_DESCRIPTOR{}) is the minimum length of the SD,
not the actual length. Use the actual length for computing the length of the
slice.
This path also removes getSecurityDescriptorLength, which is no longer used.
Fixes https://github.com/microsoft/go-winio/issues/298
Signed-off-by: Aaron Klotz <aaron@tailscale.com>
