Do not assemble zero operators.

AMDiS/src/DOFMatrix.cc

@@ -274,7 +274,8 @@ namespace AMDiS {
     std::vector<Operator*>::iterator it = operators.begin();
     std::vector<double*>::iterator factorIt = operatorFactor.begin();
     for (; it != operators.end(); ++it, ++factorIt)
-      if ((*it)->getNeedDualTraverse() == false) 
+      if ((*it)->getNeedDualTraverse() == false && 
+	  (*factorIt == NULL || **factorIt != 0.0))
 	(*it)->getElementMatrix(elInfo,	elementMatrix, *factorIt ? **factorIt : 1.0);
 
     if (factor != 1.0)