gobject: Handle ref_count==0 in notify_by_pspec

gobject: Handle ref_count==0 in notify_by_pspec

Just like g_object_notify, check for a zero ref_count in
g_object_notify_by_pspec and leave if it is 0.

This allows using functions in ->finalize() that possibly also
notify a property change on the object.  Previously,
this resulted in an error from g_object_ref.

https://bugzilla.gnome.org/show_bug.cgi?id=705570