Stop using unnecessary std::auto_ptr

No need to allocate mpMasterPages dynamically.
Thanks David Tardon for pointing it out.

Change-Id: Id91ce50b190c7807f4c59d211110c28409c244ca
Reviewed-on: https://gerrit.libreoffice.org/11484
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>