title: Java error stacktrace accumulation
body: <p>I develop smpp client, when i receive InvalidResponseException time after time, my stacktrace increase:</p>
  
  <pre><code>org.jsmpp.extra.NegativeResponseException: Negative response 0000000f found
      at org.jsmpp.session.AbstractSession.validateResponse(AbstractSession.java:162)
      at org.jsmpp.session.AbstractSession.executeSendCommand(AbstractSession.java:203)
      at org.jsmpp.session.SMPPSession.sendBind(SMPPSession.java:143)
      at org.jsmpp.session.SMPPSession.connectAndBind(SMPPSession.java:86)
      at com.sms.client.gw.Gateway.newSession(Gateway.java:107)
      at com.sms.client.gw.Gateway.getSession(Gateway.java:81)
      at com.sms.client.gw.Gateway.access$300(Gateway.java:33)
      at com.sms.client.gw.Gateway$SessionStateListenerImpl.onStateChange(Gateway.java:196)
      at org.jsmpp.session.AbstractSessionContext.fireStateChanged(AbstractSessionContext.java:69)
      at org.jsmpp.session.SMPPSessionContext.changeState(SMPPSessionContext.java:44)
      at org.jsmpp.session.AbstractSessionContext.close(AbstractSessionContext.java:58)
      at org.jsmpp.session.SMPPSessionContext.close(SMPPSessionContext.java:10)
      at org.jsmpp.session.AbstractSession.close(AbstractSession.java:141)
  </code></pre>
  
  <p>then stacktrace was increase:</p>
  
  <pre><code>org.jsmpp.extra.NegativeResponseException: Negative response 0000000f found
      at org.jsmpp.session.AbstractSession.validateResponse(AbstractSession.java:162)
      at org.jsmpp.session.AbstractSession.executeSendCommand(AbstractSession.java:203)
      at org.jsmpp.session.SMPPSession.sendBind(SMPPSession.java:143)
      at org.jsmpp.session.SMPPSession.connectAndBind(SMPPSession.java:86)
      at com.sms.client.gw.Gateway.newSession(Gateway.java:107)
      at com.sms.client.gw.Gateway.getSession(Gateway.java:81)
      at com.sms.client.gw.Gateway.access$300(Gateway.java:33)
      at com.sms.client.gw.Gateway$SessionStateListenerImpl.onStateChange(Gateway.java:196)
      at org.jsmpp.session.AbstractSessionContext.fireStateChanged(AbstractSessionContext.java:69)
      at org.jsmpp.session.SMPPSessionContext.changeState(SMPPSessionContext.java:44)
      at org.jsmpp.session.AbstractSessionContext.close(AbstractSessionContext.java:58)
      at org.jsmpp.session.SMPPSessionContext.close(SMPPSessionContext.java:10)
      at org.jsmpp.session.AbstractSession.close(AbstractSession.java:141)
      at org.jsmpp.session.SMPPSession.connectAndBind(SMPPSession.java:100)
      at com.sms.client.gw.Gateway.newSession(Gateway.java:107)
      at com.sms.client.gw.Gateway.getSession(Gateway.java:81)
      at com.sms.client.gw.Gateway.access$300(Gateway.java:33)
      at com.sms.client.gw.Gateway$SessionStateListenerImpl.onStateChange(Gateway.java:196)
      at org.jsmpp.session.AbstractSessionContext.fireStateChanged(AbstractSessionContext.java:69)
      at org.jsmpp.session.SMPPSessionContext.changeState(SMPPSessionContext.java:44)
      at org.jsmpp.session.AbstractSessionContext.close(AbstractSessionContext.java:58)
      at org.jsmpp.session.SMPPSessionContext.close(SMPPSessionContext.java:10)
      at org.jsmpp.session.AbstractSession.close(AbstractSession.java:141)
      at org.jsmpp.session.SMPPSession$EnquireLinkSender.run(SMPPSession.java:394)
  </code></pre>
  
  <p>and larger and larger. How it can be? Why stacktrace accumulate ?
  Between the response I set SMPPSessionContext variable to null, and SMPPSession to null</p>