Add null guard and wtf log to getSubId

This method causes crashes when SubscriptionController#getInstance
returns null (which it shouldn't but go figure). These crashes haven't
been seen in the wild but they can cause presubmit flakiness.

Bug: 79215402
Bug: 78359408
Test: atest FrameworksTelephonyTests # unit tests pass
(this is not sufficient to know that flakiness is gone)
Change-Id: I60dad9f348c441405890d0a1bdc20e0d5e440c8c
Merged-In: I60dad9f348c441405890d0a1bdc20e0d5e440c8c