AP_NavEKF3: fixed segfault when IMU mask covers more IMUs than GSF mask

libraries/AP_NavEKF3/AP_NavEKF3_MagFusion.cpp

@@ -246,7 +246,8 @@ void NavEKF3_core::SelectMagFusion()
             }
 
             float yawEKFGSF, yawVarianceEKFGSF;
-            bool canUseEKFGSF = yawEstimator->getYawData(yawEKFGSF, yawVarianceEKFGSF) &&
+            bool canUseEKFGSF = yawEstimator != nullptr &&
+                                yawEstimator->getYawData(yawEKFGSF, yawVarianceEKFGSF) &&
                                 is_positive(yawVarianceEKFGSF) && yawVarianceEKFGSF < sq(radians(GSF_YAW_ACCURACY_THRESHOLD_DEG));
             if (yawAlignComplete && canUseEKFGSF && !assume_zero_sideslip()) {
                 // use the EKF-GSF yaw estimator output as this is more robust than the EKF can achieve without a yaw measurement

libraries/AP_NavEKF3/AP_NavEKF3_VehicleStatus.cpp

@@ -410,7 +410,7 @@ void NavEKF3_core::detectFlight()
     if (!prevOnGround && onGround) {
         // landed so disable filter bank
         EKFGSF_run_filterbank = false;
-    } else if (!EKFGSF_run_filterbank && ((!prevInFlight && inFlight) || expectTakeoff)) {
+    } else if (yawEstimator && !EKFGSF_run_filterbank && ((!prevInFlight && inFlight) || expectTakeoff)) {
         // started flying so reset counters and enable filter bank
         EKFGSF_yaw_reset_ms = 0;
         EKFGSF_yaw_reset_request_ms = 0;