Fixed NPE when updating score

Fixed a race condition when DCT is updating the
network score while the data connection is being
disconnected. Fixed by adding null checking in
data connection.

Test: Telephony sanity tests
Bug: 132448291
Merged-In: Ibe7c59d9b50d8a9230ea72a7e441af9b207632df
Change-Id: Ibe7c59d9b50d8a9230ea72a7e441af9b207632df
(cherry picked from commit 37ed8024)