svl: fix crash if user cancels/closes master password dialog libreoffice-7-2

(regression from d7ba5614d90381d68f880ca7e7c5ef8bbb1b1c43)

Change-Id: I8bb9a967aefa2e88f05c23456a0dd1a090e1a5fb
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/133932
Tested-by: Jenkins
Reviewed-by: Michael Stahl <michael.stahl@allotropia.de>
(cherry picked from commit bbb8617ece6d946957c2eb96287081029bce530f)
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/133864
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Christian Lohmaier <lohmaier+LibreOffice@googlemail.com>