Truncate Altitude values

We know the highesst point on Earth, so any activity that
contains an altitude greater than that is definitely incorrect.

This patch truncates the altitude to the maximum possible, this
has the happy by product of curing a major mem alloc crash in QWT
when trying to plot altitudes with very large values.

Fixes #700

[cherry-picked from master cc2a0efe3e2d3931e69847a47b400d5edf7236af]