Merge pull request #4074 from pherl/mapat

Replace C++11 only method std::map::at
diff --git a/src/google/protobuf/util/field_mask_util.cc b/src/google/protobuf/util/field_mask_util.cc
index 4d0d3a4..8a41349 100644
--- a/src/google/protobuf/util/field_mask_util.cc
+++ b/src/google/protobuf/util/field_mask_util.cc
@@ -374,7 +374,7 @@
       }
     }
     if (ContainsKey(node->children, parts[i])) {
-      node = node->children.at(parts[i]);
+      node = node->children[parts[i]];
       if (field_descriptor->cpp_type() == FieldDescriptor::CPPTYPE_MESSAGE) {
         current_descriptor = field_descriptor->message_type();
       }