This wasn't previously handling the case where an interface in s2 was
removed and not present in s1, and would cause the Equal method to
incorrectly return that the states were equal.
Updates tailscale/corp#19124
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I3af22bc631015d1ddd0a1d01bfdf312161b9532d