AudioSystem: Ensure no nullptr access

Brute force nullptr check for safety. A warning is
issued if nullptr is found, but it should not occur.

Flag: EXEMPT bugfix
Test: for run in {1..100}; do (sleep 8; echo $run; adb shell pkill audioserver); done
Bug: 379427790
Change-Id: I3ba7a89ecfaf35b658de9477e3f27dc3a59352b2