Roll Protobuf r415:r423.

third_party/protobuf/src/google/protobuf/compiler/cpp/cpp_string_field.cc

Index: third_party/protobuf/src/google/protobuf/compiler/cpp/cpp_string_field.cc
diff --git a/third_party/protobuf/src/google/protobuf/compiler/cpp/cpp_string_field.cc b/third_party/protobuf/src/google/protobuf/compiler/cpp/cpp_string_field.cc
index 8d611b690ba2896f1fdb25e0424a86a4666faae4..3cec2b6ba868d45727dbf28b76179fd5ddbd0151 100644
--- a/third_party/protobuf/src/google/protobuf/compiler/cpp/cpp_string_field.cc
+++ b/third_party/protobuf/src/google/protobuf/compiler/cpp/cpp_string_field.cc
@@ -398,7 +398,8 @@ GenerateMergeFromCodedStream(io::Printer* printer) const {
       descriptor_->type() == FieldDescriptor::TYPE_STRING) {
     printer->Print(variables_,
       "::google::protobuf::internal::WireFormat::VerifyUTF8String(\n"
-      "  this->$name$(0).data(), this->$name$(0).length(),\n"
+      "  this->$name$(this->$name$_size() - 1).data(),\n"
+      "  this->$name$(this->$name$_size() - 1).length(),\n"
       "  ::google::protobuf::internal::WireFormat::PARSE);\n");
   }
 }