Copter/EKF3: altitude runaway if EK3_SRC1_POSZ = 2 (rangefinder) and rangefinder is out of range

[rmackay9]

We have two reports of Copters experiencing EKF altitude estimate runaway after taking off with EK3_SRC1_POSZ=2 (Rangefinder) and with the rangefinder out of range (e.g. reported range is less than RNGFND1_MIN)

This issue is not new to 4.6, at least one of the reports was with 4.5.7. Below shows a piece of the log where we can see the EKF altitude estimate (in red) is drifting. It should surely be tracking either the rangefinder altitude or barometer (used as a fallback)

This also appears on the AP-4.6 issues list