af-binder: Ensure validy of event loop

af-binder: Ensure validy of event loop

In rare circumstances during intialization threads should wait instead
of using a null event loop. This is because no one has needed event loop at
this stage of initialization.

Change-Id: Ie5c1f86f80d7b90234cedabb07186058e6b2d45f
Signed-off-by: José Bollo <jose.bollo@iot.bzh>