sd-event: take ref on event loop object before dispatching event sources

Idea is that all public APIs should take reference on objects that get
exposed to user-provided callbacks. We take the reference as a
protection from callbacks dropping it. We used to do this also here in
sd_event_loop(). However, in cleanup portion of f814c871 this was
accidentally dropped.

(cherry picked from commit 9f6ef467)
(cherry picked from commit a93ddddd)