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

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.

Cc: [email protected]
Fixes: c97cd0b4b54e ("usb: typec: tcpm: set initial svdm version based on pd revision")
Signed-off-by: Badhri Jagan Sridharan <[email protected]>
Reviewed-by: Heikki Krogerus <[email protected]>
Reviewed-by: Dmitry Baryshkov <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>