AP_InertialNav: fixed use of _ahrs.get_relative_position_NED() with EKF disabled

this prevents a floating point error caused by using an uninitialised
vector3 when switching between DCM and EKF control in AP_InertialNav

Pair-Programmed-With: Randy Mackay <rmackay9@yahoo.com>
AP_InertialNav: fixed use of ahrs.get_velocity with EKF disabled
Copter: AC3.2-rc12 release notes
Copter: version to AC3.2-rc12