math/big: fix nil bug in GobEncode

math/big: fix nil bug in GobEncode

Update #5305.
This handles the case where the nil pointers are inside a slice.
A top-level nil pointer is harder, maybe fundamentally broken by gob's model.
Thinking required.
However, a slice is the important case since people don't expect to be sending
top-level nils much, but they can arise easily in slices.

R=golang-dev, josharian, adg
CC=golang-dev
https://golang.org/cl/13042044