Fix oversight from 9e149c84 with spin-lock handling

Calling an external function while a pin-lock is held is a bad idea as
those are designed to be short-lived.  The stress of a first commit into
a large git history may contribute to that.

Reported-by: Andres Freund
Discussion: https://postgr.es/m/20180611164952.vmxdpdpirdtkdsz6@alap3.anarazel.de

src/backend/replication/slot.c

@@ -999,6 +999,7 @@ ReplicationSlotReserveWal(void)
 	while (true)
 	{
 		XLogSegNo	segno;
+		XLogRecPtr	restart_lsn;
 
 		/*
 		 * For logical slots log a standby snapshot and start logical decoding
@@ -1016,8 +1017,9 @@ ReplicationSlotReserveWal(void)
 			XLogRecPtr	flushptr;
 
 			/* start at current insert position */
+			restart_lsn = GetXLogInsertRecPtr();
 			SpinLockAcquire(&slot->mutex);
-			slot->data.restart_lsn = GetXLogInsertRecPtr();
+			slot->data.restart_lsn = restart_lsn;
 			SpinLockRelease(&slot->mutex);
 
 			/* make sure we have enough information to start */
@@ -1028,8 +1030,9 @@ ReplicationSlotReserveWal(void)
 		}
 		else
 		{
+			restart_lsn = GetRedoRecPtr();
 			SpinLockAcquire(&slot->mutex);
-			slot->data.restart_lsn = GetRedoRecPtr();
+			slot->data.restart_lsn = restart_lsn;
 			SpinLockRelease(&slot->mutex);
 		}