gmessages: support NULL log domain
gmessages: support NULL log domain

It's possible that a project may not define the G_LOG_DOMAIN but
still use g_log functions.  In such cases, we now crash.

This commit fixes that.

https://bugzilla.gnome.org/show_bug.cgi?id=769087

johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib