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/peer.go

@@ -92,7 +92,7 @@ func (device *Device) NewPeer(pk NoisePublicKey) (*Peer, error) {
 	// pre-compute DH
 	handshake := &peer.handshake
 	handshake.mutex.Lock()
-	handshake.precomputedStaticStatic = device.staticIdentity.privateKey.sharedSecret(pk)
+	handshake.precomputedStaticStatic, _ = device.staticIdentity.privateKey.sharedSecret(pk)
 	handshake.remoteStatic = pk
 	handshake.mutex.Unlock()