big: do not modify divisor
big: do not modify divisor

Fixes issue 1907.

R=rsc
CC=golang-dev
http://codereview.appspot.com/4527096

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