xserver: fix incorrect format and x addr maybe null

[GermanAizek]

x_server_get_address (server) may return NULL this dangerous since many functions may not check the input parameters for NULL

Affected funcs with maybe NULL parameter:

• l_debug
• xcb_connect_to_display_with_auth_info

[GermanAizek]

This cannot be fixed added check on NULL because g_strdup_printf(), g_snprintf() function belong to Glib.
l_debug fix He'll be here #451

[JPeisach]

No, like, if the server address is NULL then there is no server... so no connection should be attempted at that point