[LatestTimeSync] Fix crash when Synchronizeris started before the mes…

…sges are available. (#137)

include/message_filters/sync_policies/latest_time.hpp

@@ -250,7 +250,15 @@ struct LatestTime : public PolicyBase<M0, M1, M2, M3, M4, M5, M6, M7, M8>
     {
       bool step_change_detected = false;
       do {
-        double period = (now - prev).seconds();
+        double period = 0.0;
+        try {
+          period = (now - prev).seconds();
+        } catch (const std::runtime_error & /*e*/) {
+          // Different time sources that might happen on initialization if the messages are not yet
+          // available.
+          // std::cout << "Exception: " << e.what() << std::endl;
+          return false;
+        }
         if (period <= 0.0) {
           // multiple messages and time isn't updating
           return false;