[Ruby] reflection on a repeated field yields an incorrect value or SEGV

[qnighy]

What version of protobuf and what language are you using?
Version: 3.15.1
Language: Ruby

What operating system (Linux, Windows, ...) and version?
macOS and Linux

What runtime / compiler are you using (e.g., python version or gcc version)

What did you do?

Run the following:

require 'google/protobuf'
require 'google/protobuf/struct_pb'
p Google::Protobuf::ListValue.descriptor.lookup("values").get(Google::Protobuf::ListValue.new(values: [Google::Protobuf::Value.new(string_value: "Hello")]))

What did you expect to see

[<Google::Protobuf::Value: null_value: :NULL_VALUE, number_value: 0.0, string_value: "Hello", bool_value: false, struct_value: nil, list_value: nil>]

What did you see instead?

<Google::Protobuf::Value: >

In more complex cases, SEGV is also obeserved.

Anything else we should know about your project / environment

[haberman]

Fix in: #8330