[Logging]: Fix crash while exting GCS while logfile is playing. Inspired by
http://git.openpilot.org/cru/OPReview-571.
[Logging]: Remove cruft and fix formatting.
[Logging]: Fixes runaway processor at end of logfile playback.

Clears buffer which is being polled by
while (io->bytesAvailable() > 0), leading to 100% CPU usage.