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'

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<?xml version='1.0' encoding='utf-8'?>

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<PyFile xmlns:xhtml="http://www.w3.org/1999/xhtml" xmlns:xsd="http://www.w3.org/2001/XMLSchema">

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  <variables/>

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  <globals>

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    <xhtml:p><![CDATA[

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/*

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    Web interface is build as SVG file in Inkscape. 

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    To edit SVG file click 'Inkscape' button on the toolbar above.

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    Inkscape is a free and open-source vector graphics editor. 

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    It's not part of Beremiz and needs to be installed separately.

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*/

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]]></xhtml:p>

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  </globals>

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  <init>

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    <xhtml:p><![CDATA[

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]]></xhtml:p>

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  </init>

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  <cleanup>

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    <xhtml:p><![CDATA[

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]]></xhtml:p>

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  </cleanup>

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  <start>

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    <xhtml:p><![CDATA[

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]]></xhtml:p>

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  </start>

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  <stop>

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    <xhtml:p><![CDATA[

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]]></xhtml:p>

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  </stop>

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</PyFile>