The m5 and m6 fields were the wrong way round.

Fixes #17444.

Change-Id: I10297064f2cd09d037eac581c96a011358f70aae
Reviewed-on: https://go-review.googlesource.com/31130
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>