fix(frontends/lean/parser): avoid nontermination
fix(frontends/lean/parser): avoid nontermination

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

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