Fix TravisCI test

paddle/framework/backward.cc

@@ -133,8 +133,9 @@ std::shared_ptr<OperatorBase> BackwardRecursive(
     std::shared_ptr<OperatorBase> grad_op = OpRegistry::CreateGradOp(forwardOp);
     for (std::string& grad_input : grad_op->inputs_) {
       if (no_grad_names.count(grad_input)) {
+        // +1 for \0
         std::string prefix = grad_input.substr(
-            0, grad_input.size() - sizeof(kGradVarSuffix) / sizeof(char));
+            0, grad_input.size() - sizeof(kGradVarSuffix) / sizeof(char) + 1);
         grad_input = prefix + kZeroVarSuffix;
 
         // If part of input gradient of that operator is not calculated, fill