Check channel closed state in /events loop

When listener channel is closed, it becomes always available for
reading: select becomes an active loop which writes default-constructed
events (i.e: empty strings).

Fixes #5766.

Docker-DCO-1.1-Signed-off-by: Arnaud Porterie <arnaud.porterie@gmail.com> (github: icecrime)
Merge pull request #5806 from icecrime/5766-events_endless_newlines

Issue #5766 - Check channel closed state in /events loop