[NbtkTextureCache] Add param check guards on entry to some functions.
[NbtkTextureCache] Add param check guards on entry to some functions.

pwithnall/mx,jonnylamb/mx,jonnylamb/mx,clutter-project/mx,clutter-project/mx,pwithnall/mx,jonnylamb/mx,jonnylamb/mx,pwithnall/mx,pwithnall/mx,clutter-project/mx,clutter-project/mx