Improve fix wrong code generation if EN/ENO are used in FBD/LD/SFC

Commit c9915bc6 "Fix wrong code generation if EN/ENO are used in
FBD/LD/SFC" caused following backtrace in some cases:

File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 476, in GenerateProgram
self.GeneratePouProgram(pou_name)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 258, in GeneratePouProgram
program = pou_program.GenerateProgram(pou)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1713, in GenerateProgram
self.ComputeProgram(pou)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 997, in ComputeProgram
eno_var = self.GetUsedEno(body, connections)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 929, in GetUsedEno
for invar in blk.inputVariables.getvariable():
File "/home/developer/WorkData/PLC/beremiz/beremiz/xmlclass/xmlclass.py", line 1363, in getattrMethod
return DefaultElementClass.__getattribute__(self, name)
<type 'exceptions.AttributeError'>: 'inOutVariable' object has no attribute 'inputVariables'

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<PyFile>
  <variables/>
  <globals>
<![CDATA[
from threading import Thread

def OnConnectButton(self, event):
  def OnWiiConnected(mac_addr):
    self.label_2.SetLabel(
      "Wiimote %s connected"%mac_addr 
      if mac_addr else 
      "Wiimote connection failed !")

  def WiiConnected(mac_addr):
    wx.CallAfter(OnWiiConnected,mac_addr)

  Thread(target = Connect_Wiimote, args = (WiiConnected,)).start()
  self.label_2.SetLabel("Press wiimote 1+2")
  event.Skip()

def OnDisconnectButton(self, event):
  Disconnect_Wiimote()
  self.label_2.SetLabel("Wiimote disconnected")
  event.Skip()

]]>
  </globals>
  <init>
<![CDATA[
]]>
  </init>
  <cleanup>
<![CDATA[
]]>
  </cleanup>
  <start>
<![CDATA[
]]>
  </start>
  <stop>
<![CDATA[
]]>
  </stop>
</PyFile>