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'?>
<PyFile xmlns:xhtml="http://www.w3.org/1999/xhtml" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
  <variables/>
  <globals>
    <xhtml:p><![CDATA[
/*
    Web interface is build as SVG file in Inkscape. 
    To edit SVG file click 'Inkscape' button on the toolbar above.
    
    Inkscape is a free and open-source vector graphics editor. 
    It's not part of Beremiz and needs to be installed separately.
*/

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