fix(util/hash): relax pre-condition
fix(util/hash): relax pre-condition

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

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