igmp: force on selected interface (avoid crash *sometimes*)

Author: d-a-v

libraries/ESP8266mDNS/src/LEAmDNS_Helpers.cpp

@@ -199,7 +199,7 @@ bool MDNSResponder::_allocUDPContext(void) {
     //TODO: set multicast address (lwip_joingroup() is IPv4 only at the time of writing)
     multicast_addr.addr = DNS_MQUERY_IPV6_GROUP_INIT;
 #endif
-    if (ERR_OK == igmp_joingroup(IP4_ADDR_ANY4, ip_2_ip4(&multicast_addr))) {
+    if (ERR_OK == igmp_joingroup(&m_netif->ip_addr, ip_2_ip4(&multicast_addr))) {
         m_pUDPContext = new UdpContext;
         m_pUDPContext->ref();