Fixes a synchronization bug. Return if there's no timeout OR if there's no sync byte received.
The previous version needed both to be true at the same time to return without sending a
frame, which meant that a frame was sent if there was a timeout even if no sync byte had been
received. That, in turn, caused random problems for the Processing sketch on the other end.

The timeout functionality has changed, and it needs to be explained.
Timeouts because of lost sync bytes should be done on the 
opposite end if needed. Otherwise, the two sides get out of sync
every now and then.