net: core: Handle 'sk' being NULL in UID-based routing

It has Amit Pundir <amit.pundir@linaro.org> fix:
net: core: fix UID-based routing build

Bug: 15413527
Change-Id: Iab1fae9da6053b284591628ef1de878761b137b1
Signed-off-by: Sreeram Ramachandran <sreeram@google.com>
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>