altos: Bounds check Skytraq GPS tracking data array

altos: Bounds check Skytraq GPS tracking data array

Missing GPS serial data could cause the tracking array reset to
get skipped, causing the array to be overrun, smashing critical data
beyond the array.

This was detected using the 'altosui' flash command to program a
device from TM. Hitting the USB that hard caused TM to crash with a
mutex error (3 beeps) after the ao_gps_task structure was overwritten
with zeros.

Signed-off-by: Keith Packard <keithp@keithp.com>