this is presumably a regression from

commit 8904bf48
Author: Michael Stahl <mstahl@redhat.com>
Date:   Tue Jul 7 23:57:07 2015 +0200

    sd: dispose SdStyleSheet properly

    If it's disposed during document destruction it still retained pointers
    to SfxItemSet and SfxStyleSheetBasePool.

where it was always deleted in the dtor, but now
is optional in ::dispose

lets set bMySet to true when pSet is created like sc does, rather
try going to an unconditional delete in ::dispose

Change-Id: I214675805e513f657e51c779758106194f6d14e3