Copter-3.4: terrain following may lead to endless climb if sonar max alt is configured incorrectly

Tridge has found an issue in SITL in which the vehicle kept climbing and climbing during AUTO when it was requested to fly to some altitude (say 100m) but the range finder (whose maximum range was 50m) and the range finder didn't report that it went out of range. It's response was as if it was seeing a good value of 50m.
This situation can arise especially with analog range finders if the user simply sets the maximum range too high.
We should think of ways to reduce the chance of this happening.
Idea #1: use google earth data if the waypoint mission altitude-above-terrain is obviously higher than the range finder could ever measure (i.e. > rangefinder.max_cm).

Start by reproducing the reported behavior in SITL during an AUTO waypoint mission with a range finder capped at 50m that continues reporting 50m. Then trace the terrain-following and range-finder handling involved; done means the vehicle no longer enters an endless climb when the configured maximum range is incorrect or the sensor is out of range.

Written by the indexing model from the issue text.