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
https://golang.org/cl/4382044

src/pkg/big/int.go

@@ -337,6 +337,10 @@ func fmtbase(ch int) int {
 // 'x' (hexadecimal).
 //
 func (x *Int) Format(s fmt.State, ch int) {
+	if x == nil {
+		fmt.Fprint(s, "<nil>")
+		return
+	}
 	if x.neg {
 		fmt.Fprint(s, "-")
 	}