title: Issue with Pysnmp Trap Receiver.for SNMPv3
body: <p>PSNMP Socket is actually receiving traps but the call back function is not getting called. I have enabled the debug logging. Using Pysnmp 4.3.3</p>
  
  <pre><code>from pysnmp.entity import engine, config
  from pysnmp.carrier.asyncore.dgram import udp
  from pysnmp.entity.rfc3413 import ntfrcv
  from pysnmp.proto.api import v2c
  from pysnmp.smi import builder, view, compiler, rfc1902, error
  from pysnmp import debug 
  
  debug.setLogger(debug.Debug('all'))
  snmpEngine = engine.SnmpEngine()
  
  config.addTransport(
      snmpEngine,
      udp.domainName,
      udp.UdpTransport().openServerMode(('0.0.0.0', 1036))
  )
  
  config.addV3User(
      snmpEngine, 'user_snmp1234'
  )
  
  def cbFun(snmpEngine, stateReference, contextEngineId, contextName,
            varBinds, cbCtx):
      print "#######################Recived Notification from {} #######################".format(snmpEngine.msgAndPduDsp.getTransportInfo(stateReference)[-1][0])
      for oid, val in varBinds:
          output = rfc1902.ObjectType(rfc1902.ObjectIdentity(oid),
                                           val).resolveWithMib(mibViewController).prettyPrint()
          print output
  
  ntfrcv.NotificationReceiver(snmpEngine, cbFun)
  snmpEngine.transportDispatcher.jobStarted(1) 
  try:
      snmpEngine.transportDispatcher.runDispatcher()
  except:
      snmpEngine.transportDispatcher.closeDispatcher()
      raise
  </code></pre>
  
  <p><strong>I can see its reciving the v3 traps:</strong></p>
  
  <pre><code>2017-02-24 00:46:02,853 pysnmp: prepareDataElements: SNMPv3Message:
   msgVersion=3
   msgGlobalData=HeaderData:
    msgID=16259
    msgMaxSize=65535
    msgFlags=0x00
    msgSecurityModel=3
  
   msgSecurityParameters=0x3027040c80000f150000000000000000020100020100040d757365725f736e6d703132333404000400
   msgData=ScopedPduData:
    plaintext=ScopedPDU:
     contextEngineId=0x80000f150000000000000000
     contextName=
     data=PDUs:
      snmpV2-trap=SNMPv2TrapPDU:
       request-id=775985686
       error-status='noError'
       error-index=0
       variable-bindings=VarBindList:
        VarBind:
         name=1.3.6.1.2.1.1.3.0
         =_BindValue:
          value=ObjectSyntax:
           application-wide=ApplicationSyntax:
            timeticks-value=10000
  </code></pre>
  
  <p><strong>But at the end I am gating some error like this :</strong></p>
  
  <pre><code>2017-02-24 00:46:02,853 pysnmp: prepareDataElements: msg data msgVersion 3 msgID 16259 securityModel 3
  2017-02-24 00:46:02,854 pysnmp: processIncomingMsg: securityParameters
  00000: 30 27 04 0C 80 00 0F 15 00 00 00 00 00 00 00 00
  00016: 02 01 00 02 01 00 04 0D 75 73 65 72 5F 73 6E 6D
  00032: 70 31 32 33 34 04 00 04 00
  2017-02-24 00:46:02,854 pysnmp: processIncomingMsg: UsmSecurityParameters:
   msgAuthoritativeEngineId=0x80000f150000000000000000
   msgAuthoritativeEngineBoots=0
   msgAuthoritativeEngineTime=0
   msgUserName=user_snmp1234
   msgAuthenticationParameters=
   msgPrivacyParameters=
  
  2017-02-24 00:46:02,855 pysnmp: processIncomingMsg: cache write securityStateReference 6156330 by msgUserName user_snmp1234
  2017-02-24 00:46:02,855 pysnmp: processIncomingMsg: unsynchronized securityEngineID OctetString(hexValue='80000f150000000000000000')
  2017-02-24 00:46:02,855 pysnmp: processIncomingMsg: read from securityParams msgAuthoritativeEngineId OctetString(hexValue='80000f150000000000000000') msgUserName OctetString('user_snmp1234', subtypeSpec=ConstraintsIntersection(ConstraintsIntersection(), ValueSizeConstraint(0, 32)))
  2017-02-24 00:46:02,855 pysnmp: processIncomingMsg: unknown securityEngineID OctetString(hexValue='80000f150000000000000000') msgUserName OctetString('user_snmp1234', subtypeSpec=ConstraintsIntersection(ConstraintsIntersection(), ValueSizeConstraint(0, 32)))
  2017-02-24 00:46:02,855 pysnmp: StatusInformation: {'securityLevel': 1, 'contextName': '', 'val': Counter32(3), 'contextEngineId': SnmpEngineID(), 'oid': (1, 3, 6, 1, 6, 3, 15, 1, 1, 3, 0), 'maxSizeResponseScopedPDU': 65446, 'securityStateReference': 6156330, 'errorIndication': &lt;pysnmp.proto.errind.UnknownSecurityName object at 0x7f1c3d7cf910&gt;}
  2017-02-24 00:46:02,855 pysnmp: prepareDataElements: SM failed, statusInformation {'securityLevel': 1, 'contextName': '', 'val': Counter32(3), 'contextEngineId': SnmpEngineID(), 'oid': (1, 3, 6, 1, 6, 3, 15, 1, 1, 3, 0), 'maxSizeResponseScopedPDU': 65446, 'securityStateReference': 6156330, 'errorIndication': &lt;pysnmp.proto.errind.UnknownSecurityName object at 0x7f1c3d7cf910&gt;}
  2017-02-24 00:46:02,855 pysnmp: returnResponsePdu: PDU &lt;empty&gt;
  2017-02-24 00:46:02,855 pysnmp: prepareResponseMessage: stateReference 9434948
  2017-02-24 00:46:02,856 pysnmp: StatusInformation: {'errorIndication': &lt;pysnmp.proto.errind.LoopTerminated object at 0x7f1c3d7cf290&gt;}
  2017-02-24 00:46:02,856 pysnmp: prepareDataElements: error reported
  </code></pre>