The old code cannot handle unexported fields, it panics.
The new code returns error instead.

Fixes #17462

Change-Id: I927fc46b21d60e86cb52e84c65f2122f9159b21d
Reviewed-on: https://go-review.googlesource.com/31540
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>