internal/impl: minor refactoring

Change-Id: I1a5f4ca31fec72a39ba6690af06f4ae8576408cc
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/226897
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/codec_field.go b/internal/impl/codec_field.go
index 9497707..c00744d 100644
--- a/internal/impl/codec_field.go
+++ b/internal/impl/codec_field.go
@@ -824,5 +824,5 @@
 	if m, ok := v.Interface().(pref.ProtoMessage); ok {
 		return m
 	}
-	return legacyWrapMessage(v)
+	return legacyWrapMessage(v).Interface()
 }