fix warnings by ineffassign, with a one actual issue

In store/search.go, we would make a copy of a byte array, but then still use
the original instead of the copy. Could result in search operations not finding
messages that do have the content, but under very unlikely conditions only.

We'll keep running ineffassign with "make check", useful enough.

smtpserver/server.go

@@ -3424,18 +3424,16 @@ func (c *conn) deliver(ctx context.Context, recvHdrFor func(string) string, msgW
 							return fmt.Errorf("finding rejects mailbox: %v", err)
 						}
 
-						hasSpace := true
 						if !conf.KeepRejects && mbrej != nil {
-							var chl []store.Change
-							chl, hasSpace, err = a.d.acc.TidyRejectsMailbox(c.log, tx, mbrej)
+							chl, hasSpace, err := a.d.acc.TidyRejectsMailbox(c.log, tx, mbrej)
 							if err != nil {
 								return fmt.Errorf("tidying rejects mailbox: %v", err)
 							}
+							changes = append(changes, chl...)
 							if !hasSpace {
 								log.Info("not storing spammy mail to full rejects mailbox")
 								return nil
 							}
-							changes = append(changes, chl...)
 						}
 						if mbrej == nil {
 							nmb, chl, _, _, err := a.d.acc.MailboxCreate(tx, conf.RejectsMailbox, store.SpecialUse{})