imap-quota: Don't crash if invalid mailbox name is given to GETQUOTAROOT.
imap-quota: Don't crash if invalid mailbox name is given to GETQUOTAROOT.

jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jwm/dovecot-notmuch,jkerihuel/dovecot,jkerihuel/dovecot,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jwm/dovecot-notmuch,jkerihuel/dovecot