[NaCl SDK] IPV6_JOIN_GROUP should call JoinGroup, not LeaveGroup

Looks like a possible C&P error.  Compile-tested only.

BUG=none
TEST=none
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_nacl_sdk;master.tryserver.chromium.mac:mac_nacl_sdk;master.tryserver.chromium.win:win_nacl_sdk

Review-Url: https://codereview.chromium.org/2540513003
Cr-Commit-Position: refs/heads/master@{#434851}

native_client_sdk/src/libraries/nacl_io/socket/udp_node.cc

@@ -436,8 +436,8 @@ Error UdpNode::SetSockOptIPV6(int optname, const void* optval, socklen_t len) {
       memcpy(&sin.sin6_addr, &mreq->ipv6mr_multiaddr, sizeof(struct in6_addr));
 
       PP_Resource net_addr = SockAddrIn6ToResource(&sin, sizeof(sin));
-      int32_t error = UDPInterface()->LeaveGroup(socket_resource_, net_addr,
-                                                 PP_BlockUntilComplete());
+      int32_t error = UDPInterface()->JoinGroup(socket_resource_, net_addr,
+                                                PP_BlockUntilComplete());
       return PPERROR_TO_ERRNO(error);
     }
     case IPV6_LEAVE_GROUP: {