Fix: fix pglogrepl.SendStandbyCopyDone

Change the return value '*CopyDoneResult' to struct to avoid NPE when receive a DataRow message.

Author: zzzzwc

pglogrepl.go

@@ -671,7 +671,7 @@ type CopyDoneResult struct {
 
 // SendStandbyCopyDone sends a StandbyCopyDone to the PostgreSQL server
 // to confirm ending the copy-both mode.
-func SendStandbyCopyDone(_ context.Context, conn *pgconn.PgConn) (cdr *CopyDoneResult, err error) {
+func SendStandbyCopyDone(_ context.Context, conn *pgconn.PgConn) (cdr CopyDoneResult, err error) {
 	// I am suspicious that this is wildly wrong, but I'm pretty sure the previous
 	// code was wildly wrong too -- wttw <steve@blighty.com>
 	conn.Frontend().Send(&pgproto3.CopyDone{})