GraphicManager: fix crash when swapping out graphics

GraphicManager::ImplCheckSizeOfSwappedInGraphics() may access a deleted
GraphicObject because swapping out one GraphicObject may actually delete
other ones, by deleting SdrGrafPrimitive2D instances via
sdr::contact::ViewContactOfGraphic::flushGraphicObjects().

(regression from 0ca0202a0994c0b7c99c366fd5cafd8a655df203)

Change-Id: I94bd465d90afbfdd14da28e42de3ecdff3a9d9f9
Reviewed-on: https://gerrit.libreoffice.org/11215
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>