The size returned by insertAt falls back to the master size if needed.

Change-Id: I741b48616c6c848fc641c597cc72bf8b17e3cad4
Reviewed-on: https://gerrit.libreoffice.org/48456Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>