[PyROOT] Fix possible segfault in TTree.AsMatrix tutorial
[PyROOT] Fix possible segfault in TTree.AsMatrix tutorial

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