Shared mailboxes: Don't crash in some systems when debug logging NULL root_dir.
Shared mailboxes: Don't crash in some systems when debug logging NULL root_dir.

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