[gdm] manager: fix monitor hotplug segfault

[Ray Strode <halfline src gnome org>]

commit c229f90449277c475a15d78a697b7fa13884e69b
Author: Richard Bradfield <bradfirj fstab me>
Date:   Sun May 24 21:48:01 2015 +0100

    manager: fix monitor hotplug segfault
    
    commit e5a0e92f59e256edc6489f2234fbe54c25ba9743 introduced a way to
    find a user session associated with a display object.
    
    That function has a bug in it, where it skips every even
    registered user session because it follows the next pointer
    twice per iteration of the loop.
    
    This can cause a crash on monitor hotplug, and in other scenarios
    if there are an odd number of user sessions (since the
    terminating NULL will be even and skipped over).
    
    https://bugzilla.gnome.org/show_bug.cgi?id=749987

 daemon/gdm-manager.c |    7 +++----
 1 files changed, 3 insertions(+), 4 deletions(-)
---
diff --git a/daemon/gdm-manager.c b/daemon/gdm-manager.c
index 64f11dd..5e496f7 100644
--- a/daemon/gdm-manager.c
+++ b/daemon/gdm-manager.c
@@ -1126,11 +1126,10 @@ static GdmSession *
 get_user_session_for_display (GdmManager *self,
                               GdmDisplay *display)
 {
-        GList *node;
 
-        for (node = self->priv->user_sessions;
-             node != NULL;
-             node = node->next) {
+        GList *node = self->priv->user_sessions;
+
+        while (node != NULL) {
                 GdmSession *session = node->data;
                 GdmDisplay *candidate_display;
                 GList *next_node = node->next;