Fix endless file reading even with a set stop event

[lzhou3957]

Problem: when a dlt file always has incoming dlt messages, the file reading fails to stop even when users set the stop_reading_event. Because we dont check the status of stop_reading_event while reading dlt file returns OK, so the set stop_reading_event wont be perceived

Solution: while reading dlt file returns OK, we have another check of the status of stop_reading_event, so that the reading could be stopped accordingly