math/rand: Intn etc. should panic if their argument is <= 0.
math/rand: Intn etc. should panic if their argument is <= 0.

I am making a unilateral decision here. I could also settle for returning 0,
as long it's documented, but I argue that it's equivalent to an index
out of bounds.

Fixes issue 2892.

R=golang-dev, dsymonds, rsc
CC=golang-dev
http://codereview.appspot.com/5676079

wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old