Use proc_exit() in WalRcvWaitForStartPosition

Hi,

While working on another patch, I happened to notice that WalRcvWaitForStartPosition() calls raw exit(1). I think this
shoulduse proc_exit(1) instead, so that the normal cleanup machinery is not bypassed. 

This tiny patch just replaces exit(1) with proc_exit(1) in WalRcvWaitForStartPosition().

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/