Check the return value for negative index, to prevent crash.

Also, use at(index) when unsure about boundary condition.  That makes
it easier to detect where illegal memory access is being done.

sc/source/core/data/dpcache.cxx

@@ -807,7 +807,7 @@ SCROW ScDPCache::GetRowCount() const
 const ScDPCache::DataListType& ScDPCache::GetDimMemberValues(SCCOL nDim) const
 {
     OSL_ENSURE( nDim>=0 && nDim < mnColumnCount ," nDim < mnColumnCount ");
-    return maFields[nDim].maItems;
+    return maFields.at(nDim).maItems;
 }
 
 sal_uLong ScDPCache::GetNumberFormat( long nDim ) const

sc/source/core/data/dpdimsave.cxx

@@ -395,12 +395,15 @@ void ScDPSaveGroupDimension::AddToData( ScDPGroupTableData& rData ) const
 
 void ScDPSaveGroupDimension::AddToCache(ScDPCache& rCache) const
 {
+    long nSourceDim = rCache.GetDimensionIndex(aSourceDim);
+    if (nSourceDim < 0)
+        return;
+
     long nDim = rCache.AppendGroupField();
     SvNumberFormatter* pFormatter = rCache.GetDoc()->GetFormatTable();
 
     if (nDatePart)
     {
-        long nSourceDim = rCache.GetDimensionIndex(aSourceDim);
         fillDateGroupDimension(rCache, aDateInfo, nSourceDim, nDim, nDatePart, pFormatter);
         return;
     }
@@ -416,7 +419,6 @@ void ScDPSaveGroupDimension::AddToCache(ScDPCache& rCache) const
         }
     }
 
-    long nSourceDim = rCache.GetDimensionIndex(aSourceDim);
     const ScDPCache::DataListType& rItems = rCache.GetDimMemberValues(nSourceDim);
     {
         ScDPCache::DataListType::const_iterator it = rItems.begin(), itEnd = rItems.end();
@@ -466,6 +468,9 @@ void ScDPSaveNumGroupDimension::AddToData( ScDPGroupTableData& rData ) const
 void ScDPSaveNumGroupDimension::AddToCache(ScDPCache& rCache) const
 {
     long nDim = rCache.GetDimensionIndex(aDimensionName);
+    if (nDim < 0)
+        return;
+
     if (aDateInfo.mbEnable)
     {
         // Date grouping