Fix: On cluster signoff, make sure that the g_source gets disconnected.

With the FALSE value this didn't happen, and so spurious incoming events
could cause a segfault as the link is already destroyed, and so
s->hbchan->llc_ops in libhbclient would already be NULL but got
dereferenced.

Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>