altoslib: Correct stats for flights without a real landing

The landing time was miscomputed when the flight ended without
entering the landed state, as when the recording terminates at
apogee.

This led to an uncomputed average boost accel, so that is now checked
separately from max accel (which is done over the whole flight, not
just boost).

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