Fix use after free when returned objects hold only one ref

Fix use after free when returned objects hold only one ref

It seems that not all code expects atk_object_ref_accessible_child()
returning NULL, neither that it can return an object with only one
reference, thus the following unref in the code can cause use-after-free
eventually.

At least the chunk in impl_GetChildAtIndex() avoids runtime warning about
invalid object being passed to g_object_unref(), which happened, in this
case, when evolution returned NULL. Evolution returns objects with one
reference only often, which tries to address the other chunks here.

https://bugzilla.gnome.org/show_bug.cgi?id=781716