a small patch to fix runes split across auspicious byte boundaries

the report from andrey

	From: andrey mirtchovski <mirtchovski@gmail.com>
	To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>,
	Subject: [9fans] acme Edit bug

	Edit (or something in its chain) appears to be broken on buffer
	boundary in p9p and in plan9. To test:

	$ perl -e '{print "A"x1023;}' > t.txt # for plan9 acme replace 1023 with 8191
	$ echo [any multibyte utf rune, for example Щ] >> t.txt
	$ acme t.txt
	[ in acme, middle-swipe "Edit , |cat" without the quotes ]

for plan 9

	awk 'BEGIN{for(i = 0; i < 8191; i++) printf "a"; print "☺"}' > m

the lgtm (for p9p) from rob

	From: r@golang.org
	To: quanstro@quanstro.net, rsc@golang.org
	Cc: plan9port.codebot@gmail.com, reply@codereview-hr.appspotmail.com

	LGTM
	rsc will have to check it in though.
	thanks.

	http://codereview.appspot.com/6586067/