use move method for SetString
diff --git a/python/google/protobuf/pyext/message.cc b/python/google/protobuf/pyext/message.cc
index dc2f772..43e5896 100644
--- a/python/google/protobuf/pyext/message.cc
+++ b/python/google/protobuf/pyext/message.cc
@@ -788,11 +788,11 @@
 
   string value_string(value, value_len);
   if (append) {
-    reflection->AddString(message, descriptor, value_string);
+    reflection->AddString(message, descriptor, std::move(value_string));
   } else if (index < 0) {
-    reflection->SetString(message, descriptor, value_string);
+    reflection->SetString(message, descriptor, std::move(value_string));
   } else {
-    reflection->SetRepeatedString(message, descriptor, index, value_string);
+    reflection->SetRepeatedString(message, descriptor, index, std::move(value_string));
   }
   return true;
 }