TreeViewCol: Fix SEGV on remove/add, + memory leak

TreeViewCol: Fix SEGV on remove/add, + memory leak

Nulling priv->button in _unset_tree_view() is asymmetrical: we create
it via init(), not _set_tree_view(), so we shouldn’t null in the latter.

Worse, doing so manifests in criticals + a SEGV easily with basic use of
testtreecolumns, removing the TVC from a TV then trying to add it to one

Finally, the wrong null-out meant dispose() failed to unref the button,
so it leaked.

https://bugzilla.gnome.org/show_bug.cgi?id=728452
https://bugzilla.gnome.org/show_bug.cgi?id=788614