Resolves: tdf#91950 optional entries may not be present

here we appear to check if the preceding optional entries are of certain
types. Presumably if they are optional then its legal for them not to
exist at all, just that if they are there they should be in the right
order.

Change-Id: Id147b91d7d375d7985d901cda112c7757dc3fb19
Reviewed-on: https://gerrit.libreoffice.org/17336Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

xmloff/source/text/txtexppr.cxx

@@ -71,35 +71,42 @@ void XMLTextExportPropertySetMapper::handleElementItem(
 
     case CTF_BACKGROUND_URL:
         {
-            assert(pProperties && nIdx >= 3);
             const Any *pPos = 0, *pFilter = 0, *pTrans = 0;
             sal_uInt32 nPropIndex = rProperty.mnIndex;
 
             // these are all optional, so have to check them in order
             // note: this index order dependency is a steaming pile of manure
-            sal_uInt32 nIndex(nIdx - 1);
-            const XMLPropertyState& rFilter = (*pProperties)[nIndex];
-            if (CTF_BACKGROUND_FILTER == getPropertySetMapper()
-                    ->GetEntryContextId(rFilter.mnIndex))
+            if (nIdx)
             {
-                pFilter = &rFilter.maValue;
-                --nIndex;
+                const XMLPropertyState& rFilter = (*pProperties)[nIdx - 1];
+                if (CTF_BACKGROUND_FILTER == getPropertySetMapper()
+                        ->GetEntryContextId(rFilter.mnIndex))
+                {
+                    pFilter = &rFilter.maValue;
+                    --nIdx;
+                }
             }
 
-            const XMLPropertyState& rPos = (*pProperties)[nIndex];
-            if (CTF_BACKGROUND_POS == getPropertySetMapper()
-                    ->GetEntryContextId(rPos.mnIndex))
+            if (nIdx)
             {
-                pPos = &rPos.maValue;
-                --nIndex;
+                const XMLPropertyState& rPos = (*pProperties)[nIdx - 1];
+                if (CTF_BACKGROUND_POS == getPropertySetMapper()
+                        ->GetEntryContextId(rPos.mnIndex))
+                {
+                    pPos = &rPos.maValue;
+                    --nIdx;
+                }
             }
 
-            const XMLPropertyState& rTrans = (*pProperties)[nIndex];
-            // #99657# transparency may be there, but doesn't have to be.
-            // If it's there, it must be in the right position.
-            if( CTF_BACKGROUND_TRANSPARENCY == getPropertySetMapper()
-                  ->GetEntryContextId( rTrans.mnIndex ) )
-                pTrans = &rTrans.maValue;
+            if (nIdx)
+            {
+                const XMLPropertyState& rTrans = (*pProperties)[nIdx - 1];
+                // #99657# transparency may be there, but doesn't have to be.
+                // If it's there, it must be in the right position.
+                if( CTF_BACKGROUND_TRANSPARENCY == getPropertySetMapper()
+                      ->GetEntryContextId( rTrans.mnIndex ) )
+                    pTrans = &rTrans.maValue;
+            }
 
             pThis->maBackgroundImageExport.exportXML(
                     rProperty.maValue, pPos, pFilter, pTrans,