UPSTREAM: 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.

Bug: 150877929
Reported-by: Vladimir Yerilov <openmindead@gmail.com>
Fixes: ad74b8649bea ("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>
Change-Id: I600cfd688ed936e45f45aad409d3b166664038d4
Signed-off-by: Will McVicker <willmcvicker@google.com>
(cherry picked from commit 5f54a85db5df67df8161739a4b2f9c2b7ab219fe)