Handle unopened data writers for ran_out_of_space and friends

We used to crash when writers weren't open.  Instead ignore them.

Change-Id: I24176837d350bd0aa2a5c750222bbb4749b83b26
diff --git a/aos/events/logging/log_namer.h b/aos/events/logging/log_namer.h
index 5534131..7e58152 100644
--- a/aos/events/logging/log_namer.h
+++ b/aos/events/logging/log_namer.h
@@ -322,6 +322,7 @@
   T accumulate_data_writers(T t, BinaryOperation op) const {
     for (const std::pair<const Channel *const, DataWriter> &data_writer :
          data_writers_) {
+      if (!data_writer.second.writer) continue;
       t = op(std::move(t), data_writer.second);
     }
     if (data_writer_.writer) {