swim: fix zero division

If swim_cfg() fails during first configuration after the self
member is created, it is deleted before return. Deletion of the
member causes member update event addition to the dissemination
queue. The event TTL depends on log2(member count), where the
count is zero in the described situation. This leads to zero
division according to clang sanitizer.

The patch makes member deletion from the member table happen a
little later, so the member event registration can't ever see the
table empty.

Part of #4609