Fixes so that bw.Position is not used when NetworkStream (#349)

Only works when MemoryStream (pending) is used.

src/NATS.Client/Conn.cs

@@ -2496,7 +2496,7 @@ internal void publish(string subject, string reply, byte[] data, int offset, int
                         else
                             kickFlusher();
 
-                        if (bw.Position + count + pubProtoLen > rbsize)
+                        if (pending != null && bw.Position + count + pubProtoLen > rbsize)
                             throw new NATSReconnectBufferException("Reconnect buffer exceeded.");
                     }
                 }