don't crash on layout of ooo55381-1.sxw

The pLay here is getting joined and thus deleted in a stack of
FormatLayoutTabs so set it as unjoinable for the duration that
we expect pLay to remain valid

Change-Id: Iedfaf710022ad19c0f415cee84f70c74dd7dbd96

sw/source/core/layout/layact.cxx

@@ -1393,6 +1393,7 @@ bool SwLayAction::FormatLayout( SwLayoutFrm *pLay, bool bAddRect )
     if ( pLay->IsFootnoteFrm() ) // no LayFrms as Lower
         return bChanged;
 
+    FlowFrmJoinLockGuard aJoinGuard(pLay);
     SwFrm *pLow = pLay->Lower();
     bool bTabChanged = false;
     while ( pLow && pLow->GetUpper() == pLay )