Fixes IPv6 multicast join issue

Problem Statement:
During multicast join sequence, InternetSocket::join_multicast_group() calls InternetSocket::modify_multicast_group(). modify_multicast_group() sets up the multicast group address (i.e., mreq.imr_multiaddr) to be joined and the interface address (i.e., mreq.imr_interface) to be used for the multicast join request. The interface address is initialized with the default value, which sets the version of interface address to NSAPI_UNSPEC. This results in LWIP::setsockopt() API to attempt IPv6 multicast join on the IPv4 interface address, hence IPv6 multicast join always fails with the protocol error.

Fix:
Initialize interface address version based on the multicast address version in LWIP::setsockopt(), before attempting multicast join operation.

Author: Sathish Kumar Mani

features/lwipstack/LWIPStack.cpp

@@ -566,6 +566,14 @@ nsapi_error_t LWIP::setsockopt(nsapi_socket_t handle, int level, int optname, co
                     return NSAPI_ERROR_PARAMETER;
                 }
             } else {
+                /* Initialize the interface address type based on the multicast address version */
+                if (imr->imr_multiaddr.version == NSAPI_IPv4) {
+                    ip_addr_set_zero_ip4(&if_addr);
+                } else if (imr->imr_multiaddr.version == NSAPI_IPv6) {
+                    ip_addr_set_zero_ip6(&if_addr);
+                } else {
+                    return NSAPI_ERROR_PARAMETER;
+                }
                 ip_addr_set_any(IP_IS_V6(&if_addr), &if_addr);
             }