EKF: Fix bug in initialisation of height and magnetic field
EKF: Fix bug in initialisation of height and magnetic field

This prevents zero data being used to form the initial height and magnetic field.
Do not start sampling initial values until non-zero time values are retrieved from the buffer.

PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl