Regression from commit 49b67cdc
(tdf#96961 sw Hide Whitespace: still show whitespace on the last page,
2016-01-08), the problem was that in case a page used to be the last one
but no longer is, then its height was kept unchanged.

Fix this by invalidating the size of the old last page after the
insertion of the new last page, this way SwPageFrame::MakeAll() will be
called later to resize the page.

Change-Id: I672a70a77af5d306b1f8bd0b5ad6c2e375497124
Reviewed-on: https://gerrit.libreoffice.org/72256
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>