chore(library/vm/vm_expr): fix compiler warning
chore(library/vm/vm_expr): fix compiler warning

digama0/lean,dselsam/lean,digama0/lean,rlewis1988/lean,leanprover/lean,fpvandoorn/lean,fpvandoorn/lean,leanprover/lean,avigad/lean,leanprover-community/lean,johoelzl/lean,digama0/lean,avigad/lean,fpvandoorn/lean,leodemoura/lean,avigad/lean,leanprover-community/lean,rlewis1988/lean,avigad/lean,leodemoura/lean,leodemoura/lean,leanprover/lean,johoelzl/lean,fpvandoorn/lean,rlewis1988/lean,johoelzl/lean,fpvandoorn/lean,leanprover-community/lean,leodemoura/lean,rlewis1988/lean,digama0/lean,johoelzl/lean,dselsam/lean,digama0/lean,fpvandoorn/lean,Kha/lean,leanprover-community/lean,dselsam/lean,leanprover/lean,leanprover/lean,leodemoura/lean,dselsam/lean,johoelzl/lean,Kha/lean,leanprover/lean,Kha/lean,dselsam/lean,rlewis1988/lean,leanprover-community/lean,Kha/lean,Kha/lean,leanprover-community/lean,avigad/lean,dselsam/lean,johoelzl/lean,Kha/lean,leodemoura/lean,avigad/lean,digama0/lean,rlewis1988/lean