make.bash: exit if dist fails
The shell's -e doesn't work across "eval"; need to error-check by hand.
The recent spate of Darwin build failures pointed out that if the first
run of cmd/dist fails, we keep going. We shouldn't.

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/13098043
math/big: fix nil bug in GobEncode

Update issue 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://codereview.appspot.com/13042044