Handle empty and corrupted log parts.

We used to crash.  Now, sort them as "corrupted", and also use what we
can extract.

Corrupted files come from unsafe shutdowns.  It is impractical to be
perfect and never have any.

Change-Id: I68acc05d482c484f17ad335f4ab2054e180d8a37
diff --git a/aos/events/logging/lzma_encoder.h b/aos/events/logging/lzma_encoder.h
index 8caca1b..1b6a0fe 100644
--- a/aos/events/logging/lzma_encoder.h
+++ b/aos/events/logging/lzma_encoder.h
@@ -73,6 +73,9 @@
   // Flag that represents whether or not all the data from the file has been
   // successfully decoded.
   bool finished_ = false;
+
+  // Filename we are decompressing.
+  std::string filename_;
 };
 
 }  // namespace aos::logger