[ntuple] fix potential segfault for field visitors
[ntuple] fix potential segfault for field visitors

olifre/root,karies/root,olifre/root,root-mirror/root,karies/root,karies/root,karies/root,root-mirror/root,karies/root,root-mirror/root,olifre/root,olifre/root,karies/root,karies/root,olifre/root,root-mirror/root,olifre/root,karies/root,root-mirror/root,olifre/root,karies/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,olifre/root,olifre/root,karies/root,root-mirror/root,root-mirror/root,olifre/root,root-mirror/root,root-mirror/root