Fix crashing for the mavlink in the iridium mode

Special handling in the mavlink_open_uart function if mavlink is in the iridium mode.

src/modules/mavlink/mavlink_main.cpp

@@ -617,7 +617,7 @@ Mavlink::mavlink_open_uart(const int baud, const char *uart_name, const bool for
 		}
 	}
 
-	if (_uart_fd < 0) {
+	if (_uart_fd < 0 || _mode == MAVLINK_MODE_IRIDIUM) {
 		return _uart_fd;
 	}