Description of problem:
GDM crashes on some login failure scenarios.  This makes it harder to debug the real issue.  I've opened this issue so you can consider checking this pointer.

Version-Release number of selected component (if applicable):
gdm-3.22.3-12.el7.x86_64

How reproducible:
Currently unknown

Steps to Reproduce:
1. Upgrade from 7.3 to 7.4, and get an SELinux issue :)  pam_selinux(gdm-password:session): Unable to get valid context for USERNAME
2. Try to log in
3.

Actual results:
GDM crashes in remove_user_session because manager->priv->user_sessions is null.

Expected results:
GDM should not crash when PAM setup fails

Additional info:

Core was generated by `/usr/sbin/gdm'.
Program terminated with signal 11, Segmentation fault.
#0  0x00005622e431e872 in remove_user_session (manager=0x5622e5361e00, session=0x5622e5348100) at gdm-manager.c:1959
1959            node = g_list_find (manager->priv->user_sessions, session);



(gdb) print manager->priv->user_sessions
$2 = (GList *) 0x0



#0  0x00005622e431e872 in remove_user_session (manager=0x5622e5361e00, session=0x5622e5348100) at gdm-manager.c:1959
#1  0x00007f720de6edcc in ffi_call_unix64 () from /lib64/libffi.so.6
#2  0x00007f720de6e6f5 in ffi_call () from /lib64/libffi.so.6
#3  0x00007f720f1ec1a8 in g_cclosure_marshal_generic () from /lib64/libgobject-2.0.so.0
#4  0x00007f720f1eb968 in g_closure_invoke () from /lib64/libgobject-2.0.so.0
#5  0x00007f720f1fda7d in signal_emit_unlocked_R () from /lib64/libgobject-2.0.so.0
#6  0x00007f720f205af1 in g_signal_emit_valist () from /lib64/libgobject-2.0.so.0
#7  0x00007f720f205ddf in g_signal_emit () from /lib64/libgobject-2.0.so.0
#8  0x00005622e432766b in on_opened (worker=<optimized out>, res=<optimized out>, user_data=0x5622e535d9c0) at gdm-session.c:918
#9  0x00007f720f4b6f64 in g_task_return_now () from /lib64/libgio-2.0.so.0
#10 0x00007f720f4b760e in g_task_return () from /lib64/libgio-2.0.so.0
#11 0x00007f720f4f5eb9 in reply_cb () from /lib64/libgio-2.0.so.0
#12 0x00007f720f4b6f64 in g_task_return_now () from /lib64/libgio-2.0.so.0
#13 0x00007f720f4b760e in g_task_return () from /lib64/libgio-2.0.so.0
#14 0x00007f720f4ea9d9 in g_dbus_connection_call_done () from /lib64/libgio-2.0.so.0
#15 0x00007f720f4b6f64 in g_task_return_now () from /lib64/libgio-2.0.so.0
#16 0x00007f720f4b6f99 in complete_in_idle_cb () from /lib64/libgio-2.0.so.0
#17 0x00007f720ef110d7 in g_idle_dispatch () from /lib64/libglib-2.0.so.0
#18 0x00007f720ef144c9 in g_main_context_dispatch () from /lib64/libglib-2.0.so.0
#19 0x00007f720ef14818 in g_main_context_iterate.isra.21 () from /lib64/libglib-2.0.so.0
#20 0x00007f720ef14aea in g_main_loop_run () from /lib64/libglib-2.0.so.0
#21 0x00005622e4314914 in main (argc=1, argv=0x7ffed8fbf678) at main.c:404



journal: gnome-shell: Fatal IO error 11 (Resource temporarily unavailable) on X server :0.
journal: Name taken or bus went away - shutting down
gnome-session: gnome-session-binary[2167]: WARNING: Lost name on bus: org.gnome.SessionManager
gnome-session-binary[2167]: WARNING: Lost name on bus: org.gnome.SessionManager
journal: gnome-settings-daemon: Fatal IO error 2 (No such file or directory) on X server :0.
gdm: Child process -2167 was already dead.
kernel: traps: gdm[1445] general protection ip:55b7d246d872 sp:7ffd19f6de20 error:0 in gdm[55b7d2454000+5e000]
abrt-hook-ccpp: Process 1445 (gdm) of user 0 killed by SIGSEGV - dumping core
systemd: gdm.service: main process exited, code=dumped, status=11/SEGV
systemd: Unit gdm.service entered failed state.
systemd: Triggering OnFailure= dependencies of gdm.service.
systemd: gdm.service failed.