Fixed Config::printForDebug : if a stack element is out of bound, print ? instead

reading_machine/src/Config.cpp

@@ -136,7 +136,12 @@ void Config::printForDebug(FILE * dest) const
   for (auto & s : stack)
   {
     if (hasColIndex(idColName))
-      stackStr += getLastNotEmptyConst(idColName, s);
+    {
+      if (has(idColName, s, 0))
+        stackStr += getLastNotEmptyConst(idColName, s);
+      else
+        stackStr += "?";
+    }
     else
       stackStr += std::to_string(s);
     stackStr += ",";