Fix exitEventLoop with disable worker threads and running yield()ed tasks.
Fix exitEventLoop with disable worker threads and running yield()ed tasks.

st_workerTaskMutex was accessed, even when not initialized and caused an access violation. Another issue was that any task that join()ed another task was resumed from within the fiber that it was waiting on.

sigod/vibe.d,mfrischknecht/vibe.d,etcimon/vibe.0,veelo/vibe.d,sigod/vibe.d,Yoplitein/vibe.d,DBankov/vibe.d,jkm/vibe.d,schuetzm/vibe.d,gedaiu/vibe.d,machindertechnologies/vibe.d,Mihail-K/vibe.d,redstar/vibe.d,Yoplitein/vibe.d,AndreyMZ/vibe.d,sigod/vibe.d,yannick/vibe.d,alphaKAI/vibe.d,etcimon/vibe.0,alphaKAI/vibe.d,nathanchristenson/vibe.d,Geod24/vibe.d,etcimon/vibe.d,fast4shoot/vibe.d,nathanchristenson/vibe.d,chalucha/vibe.d,Kozzi11/vibe.d,ColdenCullen/vibe.d,nathanchristenson/vibe.d,jkm/vibe.d,alphaKAI/vibe.d,etcimon/vibe.d,dkillebrew/vibe.d,Yoplitein/vibe.d,rejectedsoftware/vibe.d