io: fix SectionReader Seek to seek backwards
io: fix SectionReader Seek to seek backwards

Fixes issue 899.

R=golang-dev, rminnich, adg
CC=golang-dev
http://codereview.appspot.com/1749041

Committer: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

apparentlymart/golang,gnanderson/go,gnanderson/go,apparentlymart/golang,ssrl/go,ssrl/go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,dsriram/golang,ssrl/go,dsriram/golang,h8liu/golang,gnanderson/go,h8liu/golang,ssrl/go,gnanderson/go,tv42/old-go,tv42/old-go,h8liu/golang,gnanderson/go,apparentlymart/golang,dsriram/golang,h8liu/golang,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,ssrl/go,pombredanne/go-deleteme,h8liu/golang,h8liu/golang,h8liu/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,tv42/old-go,ssrl/go,apparentlymart/golang,ssrl/go,dsriram/golang,gnanderson/go,tv42/old-go