AP_NavEKF3: Fix possible compass nullptr dereference

libraries/AP_NavEKF3/AP_NavEKF3_Outputs.cpp

@@ -404,6 +404,9 @@ void NavEKF3_core::getMagXYZ(Vector3f &magXYZ) const
 // return true if offsets are valid
 bool NavEKF3_core::getMagOffsets(uint8_t mag_idx, Vector3f &magOffsets) const
 {
+    if (!_ahrs->get_compass()) {
+        return false;
+    }
     // compass offsets are valid if we have finalised magnetic field initialisation, magnetic field learning is not prohibited,
     // primary compass is valid and state variances have converged
     const float maxMagVar = 5E-6f;