Check mAdnCache is not null before loading FDN.

Bug: 292467876
Test: Manual Verification:
- Enable FDN
- reboot device
- After reboot, no crashes were observed
- Performed basic functionality tests after reboot such as making calls and sending sms using the sim card.
- QA verified and informed that fix is working: b/292467876#comment18

Change-Id: Id4909881f9f6ac509c60889557652632f00a8523
(cherry picked from commit 886684aa)