RetransmitWindow: offset must be 'low', and not highest_delivered, as…

… someone might ask for retransmission of a message between low and highest_delivered

src/org/jgroups/stack/NakReceiverWindow.java

@@ -140,7 +140,7 @@ public NakReceiverWindow(Address local_addr, Address sender, Retransmitter.Retra
                     new RangeBasedRetransmitter(sender, cmd, sched) :
                     new DefaultRetransmitter(sender, cmd, sched);
 
-        xmit_table=new RetransmitTable(5, 10000, highest_delivered +1);
+        xmit_table=new RetransmitTable(5, 10000, low);
     }