fts: Fixed a crash if virtual mailbox search couldn't be optimized by backend.
fts: Fixed a crash if virtual mailbox search couldn't be optimized by backend.

LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot,LTD-Beget/dovecot