usb: typec: tcpm: Check for port partner validity before consuming it

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

usb: typec: tcpm: Check for port partner validity before consuming it

typec_register_partner() does not guarantee partner registration

to always succeed. In the event of failure, port->partner is set

to the error value or NULL. Given that port->partner validity is

not checked, this results in the following crash:

Unable to handle kernel NULL pointer dereference at virtual address xx

pc : run_state_machine+0x1bc8/0x1c08

lr : run_state_machine+0x1b90/0x1c08

..

Call trace:

run_state_machine+0x1bc8/0x1c08

tcpm_state_machine_work+0x94/0xe4

kthread_worker_fn+0x118/0x328

kthread+0x1d0/0x23c

ret_from_fork+0x10/0x20

To prevent the crash, check for port->partner validity before

derefencing it in all the call sites.