Steps to reproduce:
1. Start gdm with malloc debugging enabled on FreeBSD
2. 
3. 


Stack trace:
 #0  0x0000000803db64be in IA__g_strdup (
     str=0x59a140 '\uffff' <repeats 200 times>...) at gstrfuncs.c:90
 #1  0x0000000801538dfd in IA__gtk_tooltips_set_tip (tooltips=0x56d580,
     widget=0x593600, tip_text=0x59a140 '\uffff' <repeats 200 times>...,
     tip_private=0x0) at gtktooltips.c:355
 #2  0x000000000040d86d in gdm_login_session_init (menu=0x582200)
     at gdmlogin.c:1448
 #3  0x00000000004104d2 in gdm_login_gui_init () at gdmlogin.c:2789
 #4  0x0000000000412ea5 in main (argc=1, argv=0x7fffffffead0) at gdmlogin.c:3746

Other information:
The problem is that each dynamic session in the sessions list are initialized
with pointers that are subsequently free'd.  The attached patch corrects the
problem by making sure the pointers are copied before they are free'd.