add separator support to backward printing

src/common/packet-metadata.cc

@@ -1053,6 +1053,10 @@ PacketMetadata::Print (std::ostream &os, Buffer data, const PacketPrinter &print
             {
               break;
             }
+          if (item.prev != 0xffff)
+            {
+              os << printer.m_separator;
+            }
           current = item.prev;
         }
     }