tdf#117086 crash on deleted SwFootnoteBossFrame

use SwFrameDeleteGuard to lock pOldBoss to exist over the range
its directly accessed

Change-Id: I68af80114a380f68b86cbc78278fb9e5e87cccce
Reviewed-on: https://gerrit.libreoffice.org/58991Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

sw/source/core/layout/flowfrm.cxx

@@ -1872,6 +1872,8 @@ bool SwFlowFrame::MoveFwd( bool bMakePage, bool bPageBreak, bool bMoveAlways )
         }
     }
 
+    std::unique_ptr<SwFrameDeleteGuard> xDeleteGuard(bMakePage ? new SwFrameDeleteGuard(pOldBoss) : nullptr);
+
     bool bSamePage = true;
     SwLayoutFrame *pNewUpper =
             m_rThis.GetLeaf( bMakePage ? MAKEPAGE_INSERT : MAKEPAGE_NONE, true );
@@ -1910,6 +1912,8 @@ bool SwFlowFrame::MoveFwd( bool bMakePage, bool bPageBreak, bool bMoveAlways )
         pOldBoss = pOldBoss->FindFootnoteBossFrame( true );
         SwPageFrame* pNewPage = pOldPage;
 
+        xDeleteGuard.reset();
+
         // First, we move the footnotes.
         bool bFootnoteMoved = false;