Fix null pointer exception in KeyguardUpdateMonitor.

While handling the message 'carrier info update' in
KeyguardUpdateMonitor, there is a possibility of crash
when subscription id queried for returns null.
Add null check to avoid the NullPointerException.

Change-Id: I1a27f60c42a675b84aba9b1301f8155cf9586120
CRs-Fixed: 769954