[test-button-group] Don't crash on remove
[test-button-group] Don't crash on remove

If you try to remove a button when there aren't any buttons in the
group, it will segfault due to a NULL pointer access.

Also, check that there is an active button before trying to remove it to
fix a critical warning.

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