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>

libraries/AP_InertialNav/AP_InertialNav_NavEKF.cpp

@@ -25,7 +25,7 @@ void AP_InertialNav_NavEKF::init()
 void AP_InertialNav_NavEKF::update(float dt)
 {
     AP_InertialNav::update(dt);
-    _ahrs.get_relative_position_NED(_relpos_cm);
+    _ahrs_ekf.get_NavEKF().getPosNED(_relpos_cm);
     _relpos_cm *= 100; // convert to cm
 
     _haveabspos = _ahrs.get_position(_abspos);

libraries/AP_InertialNav/AP_InertialNav_NavEKF.h

@@ -14,9 +14,10 @@ class AP_InertialNav_NavEKF : public AP_InertialNav
 {
 public:
     // Constructor
-    AP_InertialNav_NavEKF(AP_AHRS &ahrs, AP_Baro &baro, GPS_Glitch& gps_glitch, Baro_Glitch& baro_glitch) :
+    AP_InertialNav_NavEKF(AP_AHRS_NavEKF &ahrs, AP_Baro &baro, GPS_Glitch& gps_glitch, Baro_Glitch& baro_glitch) :
         AP_InertialNav(ahrs, baro, gps_glitch, baro_glitch),
-        _haveabspos(false)
+        _haveabspos(false),
+        _ahrs_ekf(ahrs)
         {
         }
 
@@ -113,6 +114,7 @@ private:
     Vector3f _velocity_cm; // NEU
     struct Location _abspos;
     bool _haveabspos;
+    AP_AHRS_NavEKF &_ahrs_ekf;
 };
 
 #endif // __AP_INERTIALNAV_NAVEKF_H__