fix(frontends/lean/pp): do not invoke type checker on expressions containing free variables.
fix(frontends/lean/pp): do not invoke type checker on expressions
containing free variables.

This could happened when the pretty printer was used from Lua to print
nested subterms

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