Comment 2 for bug 1204975

StacktraceTop:
 start_authentication (username=0x0) at lightdm-gtk-greeter.c:149
 g_cclosure_marshal_VOID__VOID (closure=0x8ad53b8, return_value=0x0, n_param_values=1, param_values=0x8c20dc0, invocation_hint=0xbfcba390, marshal_data=0x0) at /build/buildd/glib2.0-2.30.0/./gobject/gmarshal.c:85
 g_closure_invoke (closure=0x8ad53b8, return_value=0x0, n_param_values=1, param_values=0x8c20dc0, invocation_hint=0xbfcba390) at /build/buildd/glib2.0-2.30.0/./gobject/gclosure.c:774
 signal_emit_unlocked_R (node=0x8ad5250, detail=0, instance=0x8af1680, emission_return=0x0, instance_and_params=0x8c20dc0) at /build/buildd/glib2.0-2.30.0/./gobject/gsignal.c:3272
 g_signal_emit_valist (instance=0x8af1680, signal_id=139, detail=0, var_args=0xbfcba54c "\210}\276\b\a") at /build/buildd/glib2.0-2.30.0/./gobject/gsignal.c:3003