Fix fdo#70807: Page Style method isInUse() broken

We need to check m_FirstMaster and m_FirstLeft too to verify is a style is used

Thanks a lot mst__ for pointing how to fix this!

Change-Id: Ic9c37c552893c17fba4aabcc0fd4beb7fe2550e8
Reviewed-on: https://gerrit.libreoffice.org/7295
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Olivier Hallot <olivier.hallot@edx.srv.br>