extra: make Scalar.Invert(0) return 0

We don't want panics scattered around.

extra.go

@@ -64,12 +64,8 @@ func (s *Scalar) pow2k(k int) {
 
 // Invert sets s to the inverse of a nonzero scalar v, and returns s.
 //
-// If t is zero, Invert will panic.
+// If t is zero, Invert returns zero.
 func (s *Scalar) Invert(t *Scalar) *Scalar {
-	if t.s == [32]byte{} {
-		panic("edwards25519: zero Scalar passed to Invert")
-	}
-
 	// Uses a hardcoded sliding window of width 4.
 	var table [8]Scalar
 	var tt Scalar

extra_test.go

@@ -109,6 +109,11 @@ func TestScalarInvert(t *testing.T) {
 	if err := quick.Check(invertWorks, quickCheckConfig32); err != nil {
 		t.Error(err)
 	}
+
+	zero := NewScalar()
+	if xx := NewScalar().Invert(zero); xx.Equal(zero) != 1 {
+		t.Errorf("inverting zero did not return zero")
+	}
 }
 
 func TestMultiScalarMultMatchesBaseMult(t *testing.T) {