When the buffer is empty, reset b.off to the beginning of the buffer to avoid growing unnecessarily.
When the buffer is empty, reset b.off to the beginning of the buffer
to avoid growing unnecessarily.

R=rsc
CC=golang-dev
http://codereview.appspot.com/176071

nightlyone/golang,tv42/old-go,ssrl/go,nightlyone/golang,tianon/debian-golang,dsriram/golang,tianon/debian-golang,h8liu/golang,pombredanne/go-deleteme,gnanderson/go,bdwalton/debian-golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tianon/debian-golang,ssrl/go,tianon/debian-golang,dsriram/golang,nightlyone/golang,nightlyone/golang,ssrl/go,gnanderson/go,h8liu/golang,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,ssrl/go,dsriram/golang,nightlyone/golang,bdwalton/debian-golang,ssrl/go,nightlyone/golang,pombredanne/go-deleteme,ssrl/go,apparentlymart/golang,dsriram/golang,dsriram/golang,dsriram/golang,apparentlymart/golang,ssrl/go,h8liu/golang,gnanderson/go,h8liu/golang,tianon/debian-golang,bdwalton/debian-golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,tianon/debian-golang,apparentlymart/golang,tv42/old-go,h8liu/golang,bdwalton/debian-golang,gnanderson/go,h8liu/golang,dsriram/golang,apparentlymart/golang,tianon/debian-golang,nightlyone/golang,bdwalton/debian-golang,tianon/debian-golang,pombredanne/go-deleteme,bdwalton/debian-golang,pombredanne/go-deleteme,apparentlymart/golang,h8liu/golang,tv42/old-go,apparentlymart/golang,bdwalton/debian-golang,gnanderson/go,bdwalton/debian-golang