Copter: avoid copying the DCM matrix

ArduCopter/test.pde

@@ -475,7 +475,7 @@ test_mag(uint8_t argc, const Menu::arg *argv)
             if(medium_loopCounter == 5) {
                 if (compass.read()) {
                     // Calculate heading
-                    Matrix3f m = ahrs.get_dcm_matrix();
+                    const Matrix3f &m = ahrs.get_dcm_matrix();
                     heading = compass.calculate_heading(m);
                     compass.null_offsets();
                 }