recover more gracefully from a error condition where cells are inserted into the cellmap after the last column bug 351328 r/sr=bzbarsky

layout/tables/nsCellMap.cpp

@@ -1571,10 +1571,11 @@ void nsCellMap::InsertCells(nsTableCellMap& aMap,
                             nsRect&         aDamageArea)
 {
   if (aCellFrames.Count() == 0) return;
+  NS_ASSERTION(aColIndexBefore >= -1, "index out of range");
   PRInt32 numCols = aMap.GetColCount();
   if (aColIndexBefore >= numCols) {
-    NS_ASSERTION(PR_FALSE, "bad arg in nsCellMap::InsertCellAt");
-    return;
+    NS_ERROR("Inserting instead of appending cells indicates a serious cellmap error");
+    aColIndexBefore = numCols - 1;
   }
 
   // get the starting col index of the 1st new cells