Problem: zyre testing code segfaults at zyre_event_print
Problem: zyre testing code segfaults at zyre_event_print

Solution: the stacktrace contains issues with zmsg_is, so simply test if
self->msg is null or not

awynne/zyre,zoobab/zyre,zoobab/zyre,hintjens/zyre,twhittock/zyre,keent/zyre,twhittock/zyre,twhittock/zyre,awynne/zyre,trevorbernard/zyre,zeromq/zyre,twhittock/zyre,hintjens/zyre,trevorbernard/zyre,trevorbernard/zyre,twhittock/zyre,trevorbernard/zyre,keent/zyre,zeromq/zyre,hintjens/zyre,zeromq/zyre,keent/zyre,awynne/zyre,twhittock/zyre,hintjens/zyre,zoobab/zyre,zeromq/zyre,keent/zyre,awynne/zyre,keent/zyre,hintjens/zyre,zeromq/zyre,hintjens/zyre,zoobab/zyre,zoobab/zyre,awynne/zyre,zoobab/zyre,trevorbernard/zyre,twhittock/zyre,zoobab/zyre,keent/zyre,hintjens/zyre,zeromq/zyre,zoobab/zyre,twhittock/zyre,awynne/zyre,zeromq/zyre,hintjens/zyre,trevorbernard/zyre,zeromq/zyre