From Olivier:

- Protection added in TPad::x3d in case there is no TView defined in the
  current TPad. Trying to visualize a such TPad with a 3D viewer (ogl or
  x3d) produced a segmentation fault.


git-svn-id: http://root.cern.ch/svn/root/trunk@10697 27541ba8-7e3a-0410-8455-c3a389f83636