usb: typec: tcpm: Fix NULL pointer dereference in tcpm_pd_svdm()

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

usb: typec: tcpm: Fix NULL pointer dereference in tcpm_pd_svdm()

It is possible that typec_register_partner() returns ERR_PTR on failure.

When port->partner is an error, a NULL pointer dereference may occur as

shown below.

[91222.095236][ T319] typec port0: failed to register partner (-17)

...

[91225.061491][ T319] Unable to handle kernel NULL pointer dereference

at virtual address 000000000000039f

[91225.274642][ T319] pc : tcpm_pd_data_request+0x310/0x13fc

[91225.274646][ T319] lr : tcpm_pd_data_request+0x298/0x13fc

[91225.308067][ T319] Call trace:

[91225.308070][ T319] tcpm_pd_data_request+0x310/0x13fc

[91225.308073][ T319] tcpm_pd_rx_handler+0x100/0x9e8

[91225.355900][ T319] kthread_worker_fn+0x178/0x58c

[91225.355902][ T319] kthread+0x150/0x200

[91225.355905][ T319] ret_from_fork+0x10/0x30

Add a check for port->partner to avoid dereferencing a NULL pointer.