move: avoid crash when message list is not ready

Fix the following crash:

panic: runtime error: invalid memory address or nil pointer dereference

goroutine 1 [running]:
        runtime/panic.go:770 +0x132
git.sr.ht/~rjarry/aerc/lib.(*MessageStore).Selected(...)
        git.sr.ht/~rjarry/aerc/lib/msgstore.go:709
git.sr.ht/~rjarry/aerc/commands/msg.Move.CallBack()
        git.sr.ht/~rjarry/aerc/commands/msg/move.go:178 +0x78
git.sr.ht/~rjarry/aerc/commands/msg.Move.Execute.func1()
        git.sr.ht/~rjarry/aerc/commands/msg/move.go:75 +0x90
git.sr.ht/~rjarry/aerc/lib.(*MessageStore).Move.func1()
        git.sr.ht/~rjarry/aerc/lib/msgstore.go:645 +0x138

When a move operation finishes, the callback is invoked but there is no
guarantee that the target account has any message store ready (e.g. IMAP
may be disconnected).

Only access acct.Store() in the happy code path and even there, only use
store if is is not nil.

Fixes: 40c25ca ("mv: allow to move messages across accounts")
Link: https://lists.sr.ht/~rjarry/aerc-devel/%[email protected]%3E
Link: https://lists.sr.ht/~rjarry/aerc-devel/%[email protected]%3E
Link: https://lists.sr.ht/~rjarry/aerc-devel/%[email protected]%3E
Reported-by: Alex Freudenberg <[email protected]>
Reported-by: Evin Yulo <[email protected]>
Reported-by: Maarten Aertsen <[email protected]>
Reported-by: Yuri Schaeffer <[email protected]>
Signed-off-by: Robin Jarry <[email protected]>
Acked-by: Tim Culverhouse <[email protected]>
Acked-by: Tristan Partin <[email protected]>

commands/msg/move.go

@@ -192,14 +192,6 @@ func (m Move) CallBack(
 	marker marker.Marker,
 	timeout bool,
 ) {
-	store := acct.Store()
-	sel := store.Selected()
-
-	dest := m.Folder
-	if len(m.Account) > 0 {
-		dest = fmt.Sprintf("%s in %s", m.Folder, m.Account)
-	}
-
 	switch msg := msg.(type) {
 	case *types.Done:
 		var s string
@@ -208,19 +200,24 @@ func (m Move) CallBack(
 		} else {
 			s = "%d message moved to %s"
 		}
+		dest := m.Folder
+		if len(m.Account) > 0 {
+			dest = fmt.Sprintf("%s in %s", m.Folder, m.Account)
+		}
 		if timeout {
 			s = "timed-out: only " + s
 			app.PushError(fmt.Sprintf(s, len(uids), dest))
 		} else {
 			app.PushStatus(fmt.Sprintf(s, len(uids), dest), 10*time.Second)
 		}
-		handleDone(acct, next, store)
+		if store := acct.Store(); store != nil {
+			handleDone(acct, next, store)
+		}
 	case *types.Error:
 		app.PushError(msg.Error.Error())
 		marker.Remark()
 	case *types.Unsupported:
 		marker.Remark()
-		store.Select(sel.Uid)
 		app.PushError("error, unsupported for this worker")
 	}
 }