Fix segfault on :view-message for unloaded message

This also fixes segfault on :view-message on empty directory

Signed-off-by: Tom Lebreux <tomlebreux@cock.li>

commands/account/view-message.go

@@ -17,8 +17,14 @@ func ViewMessage(aerc *widgets.Aerc, args []string) error {
 		return errors.New("Usage: view-message")
 	}
 	acct := aerc.SelectedAccount()
+	if acct.Messages().Empty() {
+		return nil
+	}
 	store := acct.Messages().Store()
 	msg := acct.Messages().Selected()
+	if msg == nil {
+		return nil
+	}
 	viewer := widgets.NewMessageViewer(aerc.Config(), store, msg)
 	aerc.NewTab(viewer, runewidth.Truncate(
 		msg.Envelope.Subject, 32, "…"))

widgets/msglist.go

@@ -138,6 +138,10 @@ func (ml *MessageList) Store() *lib.MessageStore {
 	return ml.store
 }
 
+func (ml *MessageList) Empty() bool {
+	return ml.store == nil || len(ml.store.Uids) == 0
+}
+
 func (ml *MessageList) Selected() *types.MessageInfo {
 	return ml.store.Messages[ml.store.Uids[len(ml.store.Uids)-ml.selected-1]]
 }