device: uniformly check ECDH output for zeros

For some reason, this was omitted for response messages.

Reported-by: z <dzm@unexpl0.red>
Fixes: 8c34c4c ("First set of code review patches")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/noise_test.go

@@ -24,10 +24,10 @@ func TestCurveWrappers(t *testing.T) {
 	pk1 := sk1.publicKey()
 	pk2 := sk2.publicKey()
 
-	ss1 := sk1.sharedSecret(pk2)
-	ss2 := sk2.sharedSecret(pk1)
+	ss1, err1 := sk1.sharedSecret(pk2)
+	ss2, err2 := sk2.sharedSecret(pk1)
 
-	if ss1 != ss2 {
+	if ss1 != ss2 || err1 != nil || err2 != nil {
 		t.Fatal("Failed to compute shared secet")
 	}
 }