GdmServer: take a reference to self in child watch function

The handlers for the "died" and "exited" signals on gdm server
trigger finalization of the slave by calling gdm_slave_stop().

This commit ensures these handlers hold a reference to the server
object, so the server doesn't get prematurely finalized before
the handler finishes.

https://bugzilla.gnome.org/show_bug.cgi?id=682571