bn256: fix gfp12 MulScalar

Previously MulScalar was ignoring the first parameter, which was inconsistent with gfp6 and gfp2.

Change-Id: I4f4a4ca2f07a25176ab159be684a02696f1580ba
GitHub-Last-Rev: ec40c0403614ed56cf329102eb274dc776fa3be8
GitHub-Pull-Request: golang/crypto#67
Reviewed-on: https://go-review.googlesource.com/c/154457
Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

bn256/gfp12.go

@@ -125,8 +125,8 @@ func (e *gfP12) Mul(a, b *gfP12, pool *bnPool) *gfP12 {
 }
 
 func (e *gfP12) MulScalar(a *gfP12, b *gfP6, pool *bnPool) *gfP12 {
-	e.x.Mul(e.x, b, pool)
-	e.y.Mul(e.y, b, pool)
+	e.x.Mul(a.x, b, pool)
+	e.y.Mul(a.y, b, pool)
 	return e
 }