runtime: fix crash if user sets MemProfileRate=0
runtime: fix crash if user sets MemProfileRate=0

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/5284044

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