audio: Handled openOutput error for Tunnel/LPA Usecase

- When openOutput is failed for Tunnel/LPA use case,
  mDirectTracks does not contain AudioSessionDescriptor
  for the given output and is leading to a crash
- Added NULL check to avoid the crash

CRs-Fixed: 791143

Change-Id: I0c09fa1df80b9240de6ab6de3f72419f78846a5c