[3484580] Fix a potential crash when loading materials referencing non-existent shadow caster/receiver materials
[3484580] Fix a potential crash when loading materials referencing non-existent shadow caster/receiver materials

--HG--
branch : v1-8

jhu-lcsr-forks/ogre,nezticle/ogre,nezticle/ogre,vancepym/ogre,nezticle/ogre,jhu-lcsr-forks/ogre,jhu-lcsr-forks/ogre,vancepym/ogre,nezticle/ogre,vancepym/ogre,jhu-lcsr-forks/ogre,jhu-lcsr-forks/ogre,vancepym/ogre,vancepym/ogre