Don't try to interpolate if we get an invalid target value
Don't try to interpolate if we get an invalid target value

http://bugzilla.clutter-project.org/show_bug.cgi?id=2412

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