Fix potential crash and leak in Binding.

Fix potential crash and leak in Binding.

With this patch, Binding will always correctly restore the original
binding when it becomes inactive (previously it would sometimes restore
a deleted binding). In the future we may want to consider changing to
retore the most recently set binding instead (it's currently unclear
whether either is a better model).

Change-Id: Iddde49de0815b2334ec3369963048554d1c8e105
Reviewed-by: Yunqiao Yin <charles.yin@nokia.com>