altoslib: Handle pressure values which are too low (including negative)

altoslib: Handle pressure values which are too low (including negative)

The atmospheric model didn't deal with pressures below the minimum
value correctly and crashed with accesses outside of the base_altitude
array.

Instead of returning errors, clamp pressure within the model and
return a fixed maximum altitude value of 100km.

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