fix(util/worker_queue): bug in join method
fix(util/worker_queue): bug in join method

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

fpvandoorn/lean,dselsam/lean,sp3ctum/lean,levnach/lean,soonhokong/lean-osx,leodemoura/lean,johoelzl/lean,leanprover/lean,eigengrau/lean,Kha/lean,fgdorais/lean,avigad/lean,htzh/lean,fpvandoorn/lean2,johoelzl/lean,rlewis1988/lean,Kha/lean,soonhokong/lean-windows,leodemoura/lean,leanprover-community/lean,UlrikBuchholtz/lean,soonhokong/lean-osx,leodemoura/lean,fpvandoorn/lean2,fgdorais/lean,fgdorais/lean,soonhokong/lean-windows,leanprover-community/lean,digama0/lean,javra/lean,Kha/lean,digama0/lean,avigad/lean,fgdorais/lean,rlewis1988/lean,avigad/lean,UlrikBuchholtz/lean,c-cube/lean,c-cube/lean,leanprover/lean,avigad/lean,c-cube/lean,leodemoura/lean,Kha/lean,Kha/lean,leanprover/lean,rlewis1988/lean,rlewis1988/lean,digama0/lean,javra/lean,eigengrau/lean,soonhokong/lean-osx,fgdorais/lean,javra/lean,dselsam/lean,leanprover-community/lean,c-cube/lean,avigad/lean,digama0/lean,levnach/lean,UlrikBuchholtz/lean,soonhokong/lean,dselsam/lean,dselsam/lean,fpvandoorn/lean2,htzh/lean,leanprover/lean,fpvandoorn/lean2,htzh/lean,sp3ctum/lean,rlewis1988/lean,soonhokong/lean,soonhokong/lean,soonhokong/lean-windows,eigengrau/lean,sp3ctum/lean,eigengrau/lean,soonhokong/lean-osx,leanprover-community/lean,sp3ctum/lean,digama0/lean,fgdorais/lean,soonhokong/lean-osx,htzh/lean,fpvandoorn/lean,avigad/lean,soonhokong/lean-windows,levnach/lean,levnach/lean,leanprover/lean,leodemoura/lean,c-cube/lean,UlrikBuchholtz/lean,leanprover-community/lean,johoelzl/lean,leodemoura/lean,dselsam/lean,eigengrau/lean,digama0/lean,javra/lean,fpvandoorn/lean,leanprover/lean,levnach/lean,htzh/lean,johoelzl/lean,johoelzl/lean,fpvandoorn/lean2,soonhokong/lean,UlrikBuchholtz/lean,johoelzl/lean,sp3ctum/lean,fpvandoorn/lean,fpvandoorn/lean,javra/lean,rlewis1988/lean,dselsam/lean,Kha/lean,soonhokong/lean-windows,soonhokong/lean,leanprover-community/lean,fpvandoorn/lean