fix(library/type_context): make sure code doesn't fail if local decl does not exist
fix(library/type_context): make sure code doesn't fail if local decl does not exist

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