gdbus: Fix using NULL DBusPendingCall when disconnected from D-Bus

gdbus: Fix using NULL DBusPendingCall when disconnected from D-Bus

From D-Bus documentation for dbus_connection_send_with_reply():

"Warning: if the connection is disconnected or you try to send Unix file
descriptors on a connection that does not support them, the
DBusPendingCall will be set to NULL, so be careful with this."

Fix these errors when killing D-Bus daemon with the client still
running:

process 5712: arguments to dbus_pending_call_set_notify() were
incorrect, assertion "pending != NULL" failed in file
../../dbus/dbus-pending-call.c line 596.
This is normally a bug in some application using the D-Bus library.
process 5712: arguments to dbus_pending_call_unref() were incorrect,
assertion "pending != NULL" failed in file
../../dbus/dbus-pending-call.c line 572.
This is normally a bug in some application using the D-Bus library.