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.cc b/aos/events/logging/log_reader.cc
index 0af69c7..1d8cf18 100644
--- a/aos/events/logging/log_reader.cc
+++ b/aos/events/logging/log_reader.cc
@@ -729,6 +729,7 @@
   // running until the last node.
 
   for (std::unique_ptr<State> &state : states_) {
+    CHECK(state);
     VLOG(1) << "Start time is " << state->monotonic_start_time(0)
             << " for node " << MaybeNodeName(state->node()) << "now "
             << state->monotonic_now();