CVE-2026-98747: 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 serialcoreunregisterport()

port->portdev is NULL when no port device is installed: it is cleared on teardown, and never set if registration failed before serialcoreportdeviceadd(). serialcoreunregisterport() passes it straight to serialcoregetctrldev(), which dereferences it:

KASAN: null-ptr-deref in range [0x0000000000000040-0x0000000000000047] RIP: serialcoreunregisterport Call Trace: serial8250unregisterport serial8250remove unbindstore

Return early when there is no port device, and read port->portdev under portmutex.

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