Previously, when g.p == nil, String() crashed. In other method like Add(),
a point with g.p == nil is treated as an identity element.

Besides, the following code is the only way to get an identity element
outside the library: g := bn256.G1{}. In this situation, g.p == nil.

For example, the following code will crash:

package main

import (
	"fmt"
	"golang.org/x/crypto/bn256"
)

func main() {
	g := bn256.G1{}
	fmt.Println(g.String())
}

Change-Id: Ied6f7c8197e7b79b0913c000a9cf1cf68f8188a9
GitHub-Last-Rev: 23246486a982ea54d6023726e048c74f02089f25
GitHub-Pull-Request: golang/crypto#75
Reviewed-on: https://go-review.googlesource.com/c/163118
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Run-TryBot: Filippo Valsorda <filippo@golang.org>