[fix] Closes #15423 Addon update deadlock due to race condition deleting CThread

Addon update deadlock due to race condition resulting from a CThread (CBusyWaiter) being on the stack getting destroyed prior to the actual thread exiting.

Something like this can happen any time a CThread is instantiated on the stack. This should probably be fixed with a longer term fix but this will address the immediate problem.

I'm on vacation ('holiday' for the EU inclined :-)  ) for the week (did this in the car on the way) so I might be slow to respond. @MartijnKaijser this is the deadlock I promised to fix.