fix(library/old_default_converter): avoid underflow
fix(library/old_default_converter): avoid underflow

rlewis1988/lean,fgdorais/lean,johoelzl/lean,fpvandoorn/lean,fgdorais/lean,Kha/lean,digama0/lean,soonhokong/lean-windows,fgdorais/lean,leodemoura/lean,fgdorais/lean,avigad/lean,fpvandoorn/lean,fgdorais/lean,johoelzl/lean,soonhokong/lean-windows,leodemoura/lean,dselsam/lean,soonhokong/lean-osx,soonhokong/lean-osx,leanprover-community/lean,digama0/lean,Kha/lean,Kha/lean,leanprover-community/lean,fpvandoorn/lean,leanprover/lean,fpvandoorn/lean,johoelzl/lean,leanprover/lean,avigad/lean,soonhokong/lean-windows,soonhokong/lean-osx,digama0/lean,dselsam/lean,soonhokong/lean-windows,soonhokong/lean-windows,leanprover/lean,fpvandoorn/lean,leodemoura/lean,leanprover/lean,dselsam/lean,Kha/lean,rlewis1988/lean,leanprover/lean,dselsam/lean,leanprover-community/lean,soonhokong/lean-osx,avigad/lean,leanprover-community/lean,digama0/lean,johoelzl/lean,fgdorais/lean,dselsam/lean,dselsam/lean,leodemoura/lean,johoelzl/lean,avigad/lean,avigad/lean,rlewis1988/lean,soonhokong/lean-osx,rlewis1988/lean,digama0/lean,Kha/lean,Kha/lean,rlewis1988/lean,avigad/lean,leodemoura/lean,rlewis1988/lean,leodemoura/lean,digama0/lean,leanprover-community/lean,leanprover-community/lean,leanprover/lean,johoelzl/lean,fpvandoorn/lean