Fix blatantly uninitialized variable in recent commit.

Doesn't anybody around here pay attention to compiler warnings?

src/backend/replication/walsender.c

@@ -565,7 +565,7 @@ ProcessStandbyReplyMessage(void)
 	{
 		TransactionId	nextXid;
 		uint32			nextEpoch;
-		bool			epochOK;
+		bool			epochOK = false;
 
 		GetNextXidAndEpoch(&nextXid, &nextEpoch);