AP_NavEKF2: fixed segfault when IMU mask covers more IMUs than GSF mask

libraries/AP_NavEKF2/AP_NavEKF2_VehicleStatus.cpp

@@ -397,7 +397,7 @@ void NavEKF2_core::detectFlight()
     if (!prevOnGround && onGround) {
         // landed so disable filter bank
         EKFGSF_run_filterbank = false;
-    } else if (!prevInFlight && inFlight) {
+    } else if (yawEstimator != nullptr && !prevInFlight && inFlight) {
         // started flying so reset counters and enable filter bank
         EKFGSF_yaw_reset_ms = 0;
         EKFGSF_yaw_reset_request_ms = 0;