Use IPADDR6_INIT() macro to set connecting IPv6 address

If LwIP is built with LWIP_IPV6_SCOPES, there is a 'zone' member in
struct ip6_addr which was not being initialized correctly, leading to
routing failures.

The tcp_connect() call would return ERR_RTE and we would completely fail
to report that error. All the user would see is 'Connecting to MQTT...'
over and over again.

src/AsyncTCP.cpp

@@ -729,9 +729,8 @@ bool AsyncClient::connect(IPAddress ip, uint16_t port){
 
 #if LWIP_IPV6
 bool AsyncClient::connect(IPv6Address ip, uint16_t port){
-    ip_addr_t addr;
-    addr.type = IPADDR_TYPE_V6;
-    memcpy(addr.u_addr.ip6.addr, static_cast<const uint32_t*>(ip), sizeof(uint32_t) * 4);
+    auto ipaddr = static_cast<const uint32_t*>(ip);
+    ip_addr_t addr = IPADDR6_INIT(ipaddr[0], ipaddr[1], ipaddr[2], ipaddr[3]);
 
     return _connect(addr, port);
 }