chore(util/rb_tree): fix clang warning
chore(util/rb_tree): fix clang warning

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