Fix crash if LD input contact isn't connected

Traceback (most recent call last):
File "/home/developer/WorkData/PLC/beremiz/beremiz/BeremizIDE.py", line 339, in OnMethod
obj.CTR.CallMethod('_'+meth)
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 1996, in CallMethod
getattr(self, method)()
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 1134, in _Build
IECGenRes = self._Generate_SoftPLC()
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 746, in _Generate_SoftPLC
if self._Generate_PLC_ST():
File "/home/developer/WorkData/PLC/beremiz/beremiz/ProjectController.py", line 763, in _Generate_PLC_ST
self._getIECgeneratedcodepath())
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCControler.py", line 456, in GenerateProgram
self.ProgramChunks = GenerateCurrentProgram(self, self.Project, errors, warnings)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1762, in GenerateCurrentProgram
generator.GenerateProgram()
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 478, in GenerateProgram
self.GeneratePouProgram(pou_name)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 260, in GeneratePouProgram
program = pou_program.GenerateProgram(pou)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1718, in GenerateProgram
self.ComputeProgram(pou)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1028, in ComputeProgram
self.GenerateBlock(instance, block_infos, body, None)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1144, in GenerateBlock
expression = self.ComputeExpression(body, connections, executionOrderId > 0)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1373, in ComputeExpression
paths = self.GeneratePaths(connections, body, order, to_inout)
File "/home/developer/WorkData/PLC/beremiz/beremiz/PLCGenerator.py", line 1344, in GeneratePaths
elif isinstance(result[0], list):
IndexError: list index out of range


Close #70

<?xml version='1.0' encoding='utf-8'?>
<CFile xmlns:xhtml="http://www.w3.org/1999/xhtml">
  <includes>
    <xhtml:p><![CDATA[
]]></xhtml:p>
  </includes>
  <variables>
    <variable name="TestInput" type="SINT" initial="0"/>
    <variable name="TestOutput" type="SINT"/>
  </variables>
  <globals>
    <xhtml:p><![CDATA[
volatile long Lock=0;
volatile char PtoC=1,CtoP=2;

extern long AtomicCompareExchange(long*,long, long);
extern char *PLC_ID;

int Simple_C_Call(int val){
  return val+1;
}

int Python_to_C_Call(char toC, char *fromC){
  /* Code called by python should never touch to 
     variables modified by PLC thread directly

     AtomicCompareExchange comes from 
     beremiz' runtime implementation */

  int res = 0;
  if(!AtomicCompareExchange((long*)&Lock, 0, 1)){
    PtoC=toC;
    *fromC=CtoP;
    AtomicCompareExchange((long*)&Lock, 1, 0);
    res=1;
  }
  printf("C code called by Python: toC %d fromC %d\n",toC,*fromC);
  printf("PLC_ID id %s\n",PLC_ID);
  return res;
}

int PLC_C_Call(char fromPLC, char *toPLC){
  /* PLC also have to be realy carefull not to 
     conflict with asynchronous python access */
  if(!AtomicCompareExchange((long*)&Lock, 0, 1)){
      CtoP = fromPLC;
      *toPLC = PtoC;
      AtomicCompareExchange((long*)&Lock, 1, 0);
      return 1;
  }
  return 0;
}
]]></xhtml:p>
  </globals>
  <initFunction>
    <xhtml:p><![CDATA[
]]></xhtml:p>
  </initFunction>
  <cleanUpFunction>
    <xhtml:p><![CDATA[
]]></xhtml:p>
  </cleanUpFunction>
  <retrieveFunction>
    <xhtml:p><![CDATA[
]]></xhtml:p>
  </retrieveFunction>
  <publishFunction>
    <xhtml:p><![CDATA[
if(!AtomicCompareExchange((long*)&Lock, 0, 1)){
    TestInput = CtoP + PtoC + TestOutput;
    AtomicCompareExchange((long*)&Lock, 1, 0);
}
]]></xhtml:p>
  </publishFunction>
</CFile>