usb: typec: Make sure an alt mode exist before getting its partner

Adding check to typec_altmode_get_partner() to prevent
potential NULL pointer dereference.

Reported-by: Vladimir Yerilov <openmindead@gmail.com>
Fixes: ad74b86 ("usb: typec: ucsi: Preliminary support for alternate modes")
Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/typec/bus.c

@@ -192,7 +192,7 @@ EXPORT_SYMBOL_GPL(typec_altmode_vdm);
 const struct typec_altmode *
 typec_altmode_get_partner(struct typec_altmode *adev)
 {
-	return &to_altmode(adev)->partner->adev;
+	return adev ? &to_altmode(adev)->partner->adev : NULL;
 }
 EXPORT_SYMBOL_GPL(typec_altmode_get_partner);