The NamePos value was not being set, and would default to a value
of zero. This would cause the printing logic to get confused as
to where exactly to place the "Has unexported fields" string.

A trivial package changes from

<
type A struct {
	A int // A
	B int
			// B
	// Has unexported fields.
}
>

to

<
type A struct {
	A int // A
	B int // B
	// Has unexported fields.
}
>

Fixes #12971

Change-Id: I53b7799a1f1c0ad7dcaddff83d9aaeb1d6b7823e
Reviewed-on: https://go-review.googlesource.com/16286
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>