AP_AccelCal: check return of get_calibrator

resolves compiler warning

libraries/AP_AccelCal/AP_AccelCal.cpp

@@ -56,7 +56,10 @@ void AP_AccelCal::update()
             case ACCEL_CAL_WAITING_FOR_ORIENTATION: {
                 // if we're waiting for orientation, first ensure that all calibrators are on the same step
                 uint8_t step;
-                cal = get_calibrator(0);
+                if ((cal = get_calibrator(0)) == NULL) {
+                    fail();
+                    return;
+                }
                 step = cal->get_num_samples_collected()+1;
 
                 for(uint8_t i=1 ; (cal = get_calibrator(i))  ; i++) {