SvTreeList::GetEntry: detect an out-of-bound and return NULL in that case

That's how this worked prior to stlification.

Change-Id: I4b1a9735c92efc181a2d1f47cafb04d2855fdfe2

svtools/source/contnr/treelist.cxx

@@ -1862,7 +1862,7 @@ SvTreeListEntry* SvTreeList::GetEntry( SvTreeListEntry* pParent, sal_uLong nPos
 SvTreeListEntry* SvTreeList::GetEntry( sal_uLong nRootPos ) const
 {
     SvTreeListEntry* pRet = 0;
-    if ( nEntryCount )
+    if ( nEntryCount && nRootPos < pRootItem->maChildren.size())
         pRet = &pRootItem->maChildren[nRootPos];
     return pRet;
 }