Protection against calling G4ThreadInitTool::terminateThread without having called initThread

Based on the discussion in ATLASSIM-4062, it seems that with the new version of TBB introduced
in `LCG_95` (see !21493) can sometimes try to call `G4ThreadInitTool::terminateThread` without
having first called `G4ThreadInitTool::initThread`. This causes the `G4ExHive-test` CI test
to fail at random. Adding some protection to `G4ThreadInitTool::terminateThread` to prevent
crashes in this case while the experts follow up with the TBB experts.