brontide: refer directly to the curve object in btcec's global namespace

This commit modifies the `ecdh` function within the `brontide` package
to refer directly to the global curve params object in the `bcec`
package rather than reference it from the target public key. This
changes fixes a class of panics that have been uncovered recently but
*doesn’t* yet fix the root cause.

brontide/noise.go

@@ -47,7 +47,7 @@ var (
 // the sha256 of the compressed shared point.
 func ecdh(pub *btcec.PublicKey, priv *btcec.PrivateKey) []byte {
 	s := &btcec.PublicKey{}
-	x, y := pub.Curve.ScalarMult(pub.X, pub.Y, priv.D.Bytes())
+	x, y := btcec.S256().ScalarMult(pub.X, pub.Y, priv.D.Bytes())
 	s.X = x
 	s.Y = y