[lib][trusty] Fix crash in current_trusty_app

If the current thread is not a trusty thread, return NULL instead of
crashing.

Fixes infinite crash loop on 32 bit arm kernel if a kernel thread
crashes.

Bug: 80146040
Change-Id: I3d3246a08de4e0a3c2acf085b9de56b3e2df8d3c
1 file changed