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

commit 4987daf8 upstream.

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.

Fixes: 5e1d4c49

 ("usb: typec: tcpm: Determine common SVDM Version")
Cc: stable@vger.kernel.org
Signed-off-by: Jimmy Hu <hhhuuu@google.com>
Link: https://lore.kernel.org/r/20231020012132.100960-1-hhhuuu@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>