big: don't crash when printing nil ints
big: don't crash when printing nil ints

"%#v" of a structure with *big.Int's tends to crash a lot otherwise.

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/4382044

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