AHRS: force an extra read of the compass on startup

the first read from the compass can be bad. This ensures we have a
good value when getting the initial AHRS yaw.

Thanks to Randy and Jason for the bug report!

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -325,7 +325,10 @@ AP_AHRS_DCM::drift_correction_yaw(void)
 		if (_compass->last_update != _compass_last_update) {
 			yaw_deltat = (_compass->last_update - _compass_last_update) * 1.0e-6;
 			_compass_last_update = _compass->last_update;
-			if (!_have_initial_yaw) {
+			// we force an additional compass read()
+			// here. This has the effect of throwing away
+			// the first compass value, which can be bad
+			if (!_have_initial_yaw && _compass->read()) {
 				float heading = _compass->calculate_heading(_dcm_matrix);
 				_dcm_matrix.from_euler(roll, pitch, heading);
 				_omega_yaw_P.zero();