device: reset @con_ip6_config on failure before RA

If the commit of static connection parameters fails before starting
RA, we should reset @con_ip6_config; otherwise any external update
arriving before the commit of RA parameters will remove from
@con_ip6_config all parameters not present externally, because in
update_ip6_config() we do:

  /* This function was called upon external changes. Remove the configuration
   * (addresses,routes) that is no longer present externally from the internal
   * config. This way, we don't re-add addresses that were manually removed
   * by the user. */
  if (priv->con_ip6_config)
      nm_ip6_config_intersect (priv->con_ip6_config, priv->ext_ip6_config);

Instead if @con_ip6_config is cleared it will be rebuilt from the
connection setting at the next commit.

Fixes-test: @ipv6_preserve_cached_routes

src/devices/nm-device.c

@@ -7421,8 +7421,10 @@ addrconf6_start_with_link_ready (NMDevice *self)
 	}
 
 	/* Apply any manual configuration before starting RA */
-	if (!ip6_config_merge_and_apply (self, TRUE))
+	if (!ip6_config_merge_and_apply (self, TRUE)) {
 		_LOGW (LOGD_IP6, "failed to apply manual IPv6 configuration");
+		g_clear_object (&priv->con_ip6_config);
+	}
 
 	/* XXX: These sysctls would probably be better set by the lndp ndisc itself. */
 	switch (nm_ndisc_get_node_type (priv->ndisc)) {