Fix use-after-free in GcalManager code

This was originally discovered and fixed by Milan Crha; I'm just
forward-porting his fix to the master branch. self->clients takes
ownership of this source but we don't currently have ownership here,
leading to a double free vulnerability.

Fixes #435

src/core/gcal-manager.c

@@ -269,7 +269,7 @@ on_calendar_created_cb (GObject      *source_object,
   client = gcal_calendar_get_client (calendar);
   source = gcal_calendar_get_source (calendar);
 
-  g_hash_table_insert (self->clients, source, calendar);
+  g_hash_table_insert (self->clients, g_object_ref (source), calendar);
 
   if (visible)
     {