Hint on a couple of NPE crashes during log reading.

Change-Id: Ib4bbda605b491e2889bb02ee4b10a179a4e5bdd4
Signed-off-by: Austin Schuh <austin.schuh@bluerivertech.com>
diff --git a/aos/events/logging/log_reader.h b/aos/events/logging/log_reader.h
index 6a0fcea..7a11495 100644
--- a/aos/events/logging/log_reader.h
+++ b/aos/events/logging/log_reader.h
@@ -543,6 +543,7 @@
     }
 
     monotonic_clock::time_point monotonic_now() const {
+      CHECK_NOTNULL(event_loop_);
       return event_loop_->monotonic_now();
     }