Fix segfault on :select-message for unloaded messages

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

commands/account/select-message.go

@@ -29,6 +29,9 @@ func SelectMessage(aerc *widgets.Aerc, args []string) error {
 	if acct == nil {
 		return errors.New("No account selected")
 	}
+	if acct.Messages().Empty() {
+		return nil
+	}
 	acct.Messages().Select(n)
 	return nil
 }