tree cd8765a4465ad8f29127561636a76b62f597dae4
parent 27af11f1443ae14a409bae76e57eda9ed6ba6733
author Joe Tsai <joetsai@digital-static.net> 1567755059 -0700
committer Joe Tsai <joetsai@digital-static.net> 1568054439 +0000

internal/impl: preserve nil in ProtoMessageVXOf

ProtoMessageV1(nil) now returns nil.
ProtoMessageV2(nil) now returns nil.

Note that the following continue to panic:
	MessageOf(nil)
	MessageDescriptorOf(nil)
	MessageTypeOf(nil)
It may be reasonable for them to also return nil in the future.

Change-Id: Icc14857252d844eb6d4dbfe0c248cef22023e930
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/193758
Reviewed-by: Damien Neil <dneil@google.com>