Actually manage memory in the old-style AOS logging

LeakSanitizer should be happy with it now. It's also still just as
thread-safe.

Change-Id: Id09a0349657cf4f719267b053f0ea3d8ec366256
diff --git a/aos/events/event_loop.h b/aos/events/event_loop.h
index de397e2..1e7d25b 100644
--- a/aos/events/event_loop.h
+++ b/aos/events/event_loop.h
@@ -358,7 +358,7 @@
   // Returns the name of the underlying queue.
   const Channel *channel() const { return sender_->channel(); }
 
-  operator bool() { return sender_ ? true : false; }
+  operator bool() const { return sender_ ? true : false; }
 
   // Returns the time_points that the last message was sent at.
   aos::monotonic_clock::time_point monotonic_sent_time() const {