Enforce header validity when writing log files

We don't support no UUID but a local + remote timestamp in the log file
header when reading.  James reports a log file which violates this.
Let's make those impossible and point the smoking gun better at the
problem.

Change-Id: I63a256f8845d1e3a4f3cea4528af4451a7945d4b
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/aos/events/logging/log_namer.cc b/aos/events/logging/log_namer.cc
index dc71348..1383aed 100644
--- a/aos/events/logging/log_namer.cc
+++ b/aos/events/logging/log_namer.cc
@@ -293,6 +293,17 @@
               state.size(), &oldest_local_unreliable_monotonic_timestamps);
 
   for (size_t i = 0; i < state.size(); ++i) {
+    if (state[i].boot_uuid == UUID::Zero()) {
+      CHECK_EQ(state[i].oldest_remote_monotonic_timestamp,
+               monotonic_clock::max_time);
+      CHECK_EQ(state[i].oldest_local_monotonic_timestamp,
+               monotonic_clock::max_time);
+      CHECK_EQ(state[i].oldest_remote_unreliable_monotonic_timestamp,
+               monotonic_clock::max_time);
+      CHECK_EQ(state[i].oldest_local_unreliable_monotonic_timestamp,
+               monotonic_clock::max_time);
+    }
+
     oldest_remote_monotonic_timestamps[i] =
         state[i].oldest_remote_monotonic_timestamp.time_since_epoch().count();
     oldest_local_monotonic_timestamps[i] =