bn256: fix String() method

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

Author: SadPencil

Diff for: bn256/bn256.go

@@ -54,6 +54,9 @@ func RandomG1(r io.Reader) (*big.Int, *G1, error) {
 }
 
 func (e *G1) String() string {
+	if e.p == nil {
+		e.p = newCurvePoint(nil)
+	}
 	return "bn256.G1" + e.p.String()
 }
 
@@ -175,6 +178,9 @@ func RandomG2(r io.Reader) (*big.Int, *G2, error) {
 }
 
 func (e *G2) String() string {
+	if e.p == nil {
+		e.p = newTwistPoint(nil)
+	}
 	return "bn256.G2" + e.p.String()
 }
 
@@ -277,8 +283,11 @@ type GT struct {
 	p *gfP12
 }
 
-func (g *GT) String() string {
-	return "bn256.GT" + g.p.String()
+func (e *GT) String() string {
+	if e.p == nil {
+		e.p = newGFp12(nil)
+	}
+	return "bn256.GT" + e.p.String()
 }
 
 // ScalarMult sets e to a*k and then returns e.