math/big: don't clobber shared underlying array in pow5 computation

Rearranged code slightly to make lifetime of underlying array of
pow5 more explicit in code.

Fixes #31184.

Change-Id: I063081f0e54097c499988d268a23813746592654
Reviewed-on: https://go-review.googlesource.com/c/go/+/170641


Reviewed-by: Filippo Valsorda <filippo@golang.org>