core: reset GList pointers to NULL when necessary

When calling g_list_free_full() to free a GList in dispose(), it is
necessary to reset the GList pointer to NULL as dispose() may be called
more than once.

src/mm-call-list.c

@@ -395,6 +395,7 @@ dispose (GObject *object)
 
     g_clear_object (&self->priv->modem);
     g_list_free_full (self->priv->list, g_object_unref);
+    self->priv->list = NULL;
 
     G_OBJECT_CLASS (mm_call_list_parent_class)->dispose (object);
 }