fix(frontends/lean/server): crash when INFO is invoked before VISIT/LOAD
fix(frontends/lean/server): crash when INFO is invoked before VISIT/LOAD

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

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