handle crashes after group invites

[zugz]

Suppose A invites B to a group, B accepts, but then A crashes without saving.
B then saves and leaves the group. B has no friend other than A in the group.
Then once A and B restart, B will not be able to rejoin the group because A
does not consider B to be an offline peer in the group. Worse, A will not be
able to reinvite B to the group -- B will ignore the invitation because B
considers themself to be in the group.