Handle invalid indexes for TabView.getTab

Return undefined if ListModel.get returned undefined instead
of throwing an exception.

Change-Id: I38cbd21472e8c409390e8269e5fa0a800106392e
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Merge remote-tracking branch 'origin/stable' into wip/tizen

Change-Id: I6bf9c237bd03f06833ad51bfdc0d63ad98e913cd