Linux: serial: core: fix NULL pointer dereference in serial_core_unregister_port()

In the Linux kernel, the following vulnerability has been resolved:

serial: core: fix NULL pointer dereference in serial_core_unregister_port()

port->port_dev is NULL when no port device is installed: it is cleared on teardown, and never set if registration failed before serial_core_port_device_add(). serial_core_unregister_port() passes it straight to serial_core_get_ctrl_dev(), which dereferences it:

KASAN: null-ptr-deref in range [0x0000000000000040-0x0000000000000047] RIP: serial_core_unregister_port Call Trace: serial8250_unregister_port serial8250_remove unbind_store

Return early when there is no port device, and read port->port_dev under port_mutex.

Also clear port->port_dev on the serial_core_register_port() error path, where the port device has already been removed.