mail_thread_init(): Don't assert-crash when checking if we can use a cached thread tree.
mail_thread_init(): Don't assert-crash when checking if we can use a cached thread tree.

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