Fix: fix pglogrepl.SendStandbyCopyDone

[zzzzwc]

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

[jackc]

Would it be better to instantiate a CopyDoneResult internally to resolve the bug rather than change the signature of the function? In theory, that's a breaking change... though I suppose no one could be relying on it because the current code crashes... 🤔