math/big: fix destination leak into result value
This code would panic:
z := big.NewInt(1)
z.SetBit(big.NewInt(0), 2, 1)
if z.Cmp(big.NewInt(1<<2)) != 0 {
        panic("fail")
}

R=rsc, gri
CC=golang-dev
https://golang.org/cl/5437081
2 files changed