add g_return_if_fail() guards to public methods
add g_return_if_fail() guards to public methods

get_progress() and set_progress() are public methods and should be
guarded by a g_return_if_fail() macro.

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