lightdm-gtk-greeter segv with unknown config options

It seems that lightdm-gtk-greeter segfaults if you have an unknown option in your configuration file.

Core was generated by `/usr/sbin/lightdm-gtk-greeter'.
Program terminated with signal SIGSEGV, Segmentation fault.
#0 0x00007f0c4fd418a9 in config_load_from_file (config=0x2222100, path=0x23127d0 "/etc/lightdm/lightdm.conf", messages=0x0, error=0x7fff572556b0)
    at configuration.c:106
106 *messages = g_list_append (*messages, g_strdup_printf (" [%s] contains unknown option %s", group, keys[j]));
[Current thread is 1 (Thread 0x7f0c51b189c0 (LWP 28281))]
(gdb) p group
$1 = (gchar *) 0x2265c10 "LightDM"
(gdb) p keys[j]
$2 = (gchar *) 0x21ecd00 "display-setup-script"
(gdb) p messages
$3 = (GList **) 0x0
(gdb) bt
#0 0x00007f0c4fd418a9 in config_load_from_file (config=0x2222100, path=0x23127d0 "/etc/lightdm/lightdm.conf", messages=0x0, error=0x7fff572556b0)
    at configuration.c:106
#1 0x00007f0c4fd41fe6 in config_load_from_standard_locations (config=0x2222100, config_path=0x0, messages=0x0) at configuration.c:241
#2 0x00007f0c4fd3f029 in update_sessions () at session.c:234
#3 0x00007f0c4fd3f0f3 in lightdm_get_sessions () at session.c:269
#4 0x000000000040c972 in main ()

So it looks like a NULL message list arg is passed to config_load_from_file() while the function expects non-NULL.