fix for NPE on resetting the bound texture.
fix for NPE on resetting the bound texture.

mono/CocosSharp,haithemaraissia/CocosSharp,TukekeSoft/CocosSharp,netonjm/CocosSharp,haithemaraissia/CocosSharp,mono/cocos2d-xna,netonjm/CocosSharp,MSylvia/CocosSharp,zmaruo/CocosSharp,hig-ag/CocosSharp,mono/CocosSharp,hig-ag/CocosSharp,MSylvia/CocosSharp,zmaruo/CocosSharp,mono/cocos2d-xna,TukekeSoft/CocosSharp