support comparison of message embed structs

[tamayika]

closes golang/protobuf#1638

This fixes panics on comparison of message embed structs.
Previously, message embed struct is treated as message.
There were two problems.

• When embed message is nil, panics on proto reflection.
• When embed message is not nil but other field is different, cmp.Equal return true.

[puellanivis]

Thank you for your interest into contributing to this project.

Unfortunately this project does not accept GitHub pull requests as the source-of-truth for this project is hosted at https://go.googlesource.com/protobuf. This project page on GitHub is a mirror of that other repository.

If you would like to contribute to this project, please follow the contribution guidelines for instructions on how to send a change. If the change you'd like to make is more substantial or introduces any new features, then it should first be discussed on the issue tracker.