Resolves: tdf#100813 crash during pagination of particular docx

Change-Id: Id2c99cc6c5fe4c3a5bcf3c0b3f16b603cdd46239
diff --git a/sw/source/core/layout/flowfrm.cxx b/sw/source/core/layout/flowfrm.cxx
index 44e8af0..3e41871 100644
--- a/sw/source/core/layout/flowfrm.cxx
+++ b/sw/source/core/layout/flowfrm.cxx
@@ -2017,6 +2017,9 @@ bool SwFlowFrame::MoveBwd( bool &rbReformat )
    }

    SwFootnoteBossFrame * pOldBoss = m_rThis.FindFootnoteBossFrame();
    if (!pOldBoss)
        return false;

    SwPageFrame * const pOldPage = pOldBoss->FindPageFrame();
    SwLayoutFrame *pNewUpper = nullptr;
    bool bCheckPageDescs = false;