Clear the buffer byte between ATB and FTB
Clear the buffer byte between ATB and FTB

This ensures it has a known start state. An unknown state risks it
looking like a tail block similar to the problem fixed by #5245.

Fixes #5305

adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython,adafruit/circuitpython