gmain: Handle case where source id overflows

0 is not a valid source id, but for long-lived programs that rapidly
create/destroy sources, it's possible for the source id to overflow.
We should handle this, because the documentation implies we will.

https://bugzilla.gnome.org/show_bug.cgi?id=687098

glib/glib-private.c

@@ -40,7 +40,8 @@ glib__private__ (void)
 
     g_get_worker_context,
 
-    g_check_setuid
+    g_check_setuid,
+    g_main_context_new_with_next_id
   };
 
   return &table;