worker_spi: Initialize bgw_notify_pid in all cases.

Commit 090d0f2050647958865cb495dff74af7257d2bb4 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.