Reuse temporary slice to avoid extra allocations
(originally done correctly by remyoudompheng@gmail.com
in https://golang.org/cl/6345075/).

benchmark           old ns/op    new ns/op    delta
BenchmarkHilbert      6252790      6262304   +0.15%
BenchmarkMul         45827438     45301002   -1.15%

R=r
CC=golang-dev
https://golang.org/cl/6346097