Section 2.2 of the referenced spec http://www.xml.com/axml/testaxml.htm
defines 0xD7FF as a (sub)range boundary, not 0xDF77.

Fixes #25172

Change-Id: Ic5a3328cd46ef6474b8e93c4a343dcfba0e6511f
Reviewed-on: https://go-review.googlesource.com/109495
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>