Created attachment 7004 Remove g_mem_is_system_malloc Ubuntu 16.04 is shipping with GLib 2.51 currently. However, in GLib 2.46, g_mem_is_system_malloc was deprecated, since it always returns TRUE. (https://developer.gnome.org/glib/stable/glib-Memory-Allocation.html#g-mem-is-system-malloc) So it should no longer be used in thunar-session-client.c.
Thanks for pointing this out, pushed your fix to master and xfce-4.12 branches.