btl tcp: Fix compile error in IPv6

In 457f058 I broke the TCP BTL with --enable-ipv6.  This patch
fixes the compile error, so IPv6 works again.

Fixed #5996

Signed-off-by: Brian Barrett <bbarrett@amazon.com>

opal/mca/btl/tcp/btl_tcp_endpoint.c

@@ -742,7 +742,7 @@ static int mca_btl_tcp_endpoint_start_connect(mca_btl_base_endpoint_t* btl_endpo
     }
 #if OPAL_ENABLE_IPV6
     if (endpoint_addr.ss_family == AF_INET6) {
-        assert(NULL != &btl_endpoint->endpoint_btl->tcp_ifaddr_6);
+        assert(NULL != &btl_endpoint->endpoint_btl->tcp_ifaddr);
         if (bind(btl_endpoint->endpoint_sd, (struct sockaddr*) &btl_endpoint->endpoint_btl->tcp_ifaddr,
                  sizeof(struct sockaddr_in6)) < 0) {
             BTL_ERROR(("bind on local address (%s:%d) failed: %s (%d)",