MAVLink app: Use right lookup function

src/modules/mavlink/mavlink_parameters.cpp

@@ -130,7 +130,7 @@ MavlinkParametersManager::handle_message(const mavlink_message_t *msg)
 
 				} else {
 					/* when index is >= 0, send this parameter again */
-					send_param(param_for_index(req_read.param_index));
+					send_param(param_for_used_index(req_read.param_index));
 				}
 			}
 			break;
@@ -192,6 +192,7 @@ MavlinkParametersManager::send(const hrt_abstime t)
 		/* look for the first parameter which is used */
 		param_t p;
 		do {
+			/* walk through all parameters, including unused ones */
 			p = param_for_index(_send_all_index);
 			_send_all_index++;
 		} while (p != PARAM_INVALID && !param_used(p));