CVE-2024-50058 serial: protect uart_port_dtr_rts() in uart_shutdown() too

In the Linux kernel, the following vulnerability has been resolved: serial: protect uartportdtrrts in uartshutdown too Commit af224ca2df29 serial: core: Prevent unsafe uart port access, part 3 added few uport == NULL checks. It added one to uartshutdown, so the commit assumes, uport can be NULL i...