crashtesting: ooo55266-3.ods some members are nullptr

Crashes since commit 6481bda6

    tdf#121612: update pivot caches during saving to XLSX

without refresh, pivot cache export was skipped entirely

Change-Id: Ic68d4974d5cb9278d61938a59aa8f1a01b920f1f
Reviewed-on: https://gerrit.libreoffice.org/64903
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>

sc/source/core/data/dpgroup.cxx

@@ -1023,7 +1023,11 @@ long ScDPGroupTableData::Compare(long nDim, long nDataId1, long nDataId2)
 {
     if ( getIsDataLayoutDimension(nDim) )
         return 0;
-    return ScDPItemData::Compare( *GetMemberById(nDim,  nDataId1),*GetMemberById(nDim,  nDataId2) );
+    const ScDPItemData* rItem1 = GetMemberById(nDim, nDataId1);
+    const ScDPItemData* rItem2 = GetMemberById(nDim, nDataId2);
+    if (rItem1 == nullptr || rItem2 == nullptr)
+        return 0;
+    return ScDPItemData::Compare( *rItem1,*rItem2);
 }
 
 #if DUMP_PIVOT_TABLE