The type of the OffPtr for the first field was incorrect. It should
have been a pointer to the field type, rather than the field
type itself.

Fixes #19475.

Change-Id: I3960b404da0f4bee759331126cce6140d2ce1df7
Reviewed-on: https://go-review.googlesource.com/37869
Run-TryBot: Michael Munday <munday@ca.ibm.com>
Reviewed-by: Keith Randall <khr@golang.org>