I2CP: Fix external I2CP broken when session ID is 0

one chance in 64K, broken since 2005
don't revert previous attempted fix using synching,
that's still a good idea

core/java/src/net/i2p/data/i2cp/MessagePayloadMessage.java

@@ -125,7 +125,7 @@ public class MessagePayloadMessage extends I2CPMessageImpl {
      */
     @Override
     public synchronized void writeMessage(OutputStream out) throws I2CPMessageException, IOException {
-        if (_sessionId <= 0)
+        if (_sessionId < 0)
             throw new I2CPMessageException("Unable to write out the message, as the session ID has not been defined");
         if (_messageId < 0)
             throw new I2CPMessageException("Unable to write out the message, as the message ID has not been defined");