Add checks for invalid indices in .obj file loading and return 0 instead of crashing when that happens. 
This applies (slightly modified) patch #296 from patchtracker. Thx @mbohnen for report and patch.


git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4983 dfc29bdd-3216-0410-991c-e03cc46cb475