worker_spi: Initialize bgw_notify_pid in all cases.

Commit 090d0f20 added new code showing
how it can be useful to set bgw_notify_pid to a non-zero value, but it
failed to make sure that the existing call to RegisterBackgroundWorker
initialized the new field at all.

Report and patch by Shigeru Hanada.

contrib/worker_spi/worker_spi.c

@@ -348,6 +348,7 @@ _PG_init(void)
 	worker.bgw_start_time = BgWorkerStart_RecoveryFinished;
 	worker.bgw_restart_time = BGW_NEVER_RESTART;
 	worker.bgw_main = worker_spi_main;
+	worker.bgw_notify_pid = 0;
 
 	/*
 	 * Now fill in worker-specific data, and do the actual registrations.