EKF: Ensure mag field state covariance data is always available for re-use

Prevents use of  _saved_mag_ef_covmat and _saved_mag_bf_variance before being initialised or when stale.

EKF/ekf.h

@@ -674,6 +674,9 @@ private:
 	// Argument is additional yaw variance in rad**2
 	void increaseQuatYawErrVariance(float yaw_variance);
 
+	// save mag field state covariance data for re-use
+	void save_mag_cov_data();
+
 	// uncorrelate quaternion states from other states
 	void uncorrelateQuatStates();