Problem: zyre_log.c:121: zyre_log_info: Assertion `self' failed.

Caused when sending JOIN/LEAVE commands to a node that isn't yet
started (so has no logger).

Solution: do not try to log these commands if the node wasn't
started yet.

Fixes #197.
Merge pull request #199 from hintjens/master

Problem: zyre_log.c:121: zyre_log_info: Assertion `self' failed.