fix nil pointer deref when importing a message that the junkfilter could not parse (e.g. malformed header)

import does its own batched junkfilter training, so the deliver function
doesn't have to do it one message at a time, writing the updated filter each
time. however, if the message cannot be parsed, it isn't trained during import,
and deliver would try to train it again. it would try to open the junk filter
to do so, but that would fail because the import function already has the
junkfilter open (and the timeout is reached). a missing error check would
continue with a nil junkfilter, resulting in the nil pointer deref.

this adds the missing error check, and makes sure the deliver function does not
also try to train unparseable imported messages.

report from Jens Hilligsøe

store/account.go

@@ -671,14 +671,12 @@ func (a *Account) DeliverX(log *mlog.Log, tx *bstore.Tx, m *Message, msgFile *os
 		xcheckf(err, "sync directory")
 	}
 
-	if notrain && m.NeedsTraining() {
-		// If this ever happens, hopefully we'll get bug reports about it.
-		log.Error("deliver of message that unexpectedly needs training", mlog.Field("messageid", m.ID), mlog.Field("trainedjunk", m.TrainedJunk), mlog.Field("flags", m.Flags))
+	if !notrain && m.NeedsTraining() {
+		l := []Message{*m}
+		err = a.RetrainMessages(log, tx, l, false)
+		xcheckf(err, "training junkfilter")
+		*m = l[0]
 	}
-	l := []Message{*m}
-	err = a.RetrainMessages(log, tx, l, false)
-	xcheckf(err, "training junkfilter")
-	*m = l[0]
 }
 
 // write contents of r to new file dst, for delivering a message.

store/train.go

@@ -2,6 +2,7 @@ package store
 
 import (
 	"errors"
+	"fmt"
 	"os"
 	"path/filepath"
 
@@ -63,6 +64,8 @@ func (a *Account) RetrainMessages(log *mlog.Log, tx *bstore.Tx, msgs []Message,
 			if err != nil && errors.Is(err, ErrNoJunkFilter) {
 				// No junk filter configured. Nothing more to do.
 				return nil
+			} else if err != nil {
+				return fmt.Errorf("open junk filter: %v", err)
 			}
 			defer func() {
 				if jf != nil {