shell: unref correct proxy on destroy

gsm-shell.c tries to automatically clear it's reference to the
EndSessionDialog proxy when the proxy is destroyed.

It accidentally unrefs the wrong object though.  This commit
fixes that by changing the open coded unref+nullify to g_clear_object

I believe this will address this crasher:

https://retrace.fedoraproject.org/faf/problems/1214348/

https://bugzilla.gnome.org/show_bug.cgi?id=709221
Updated Serbian translation