fix an error

python/paddle/v2/fluid/backward.py

@@ -53,7 +53,8 @@ def _is_all_in_set_(cands, s):
 
 
 def _strip_grad_suffix_(name):
-    return name[:name.find(core.grad_var_suffix())]
+    pos = name.find(core.grad_var_suffix())
+    return name[:pos] if pos != -1 else name
 
 
 def _append_grad_suffix_(name):
@@ -139,7 +140,7 @@ def _append_backward_ops_(target,
     to_insert = []
     for idx, op_desc in enumerate(grad_op_descs):
         for arg in op_desc.input_arg_names():
-            if arg in no_grad_set[block.idx]:
+            if core.grad_var_suffix() in arg and arg in no_grad_set[block.idx]:
                 to_insert.append((arg, idx))
     for ele in reversed(to_insert):
         arg = ele[0]