logind: fix crash in logind on user-specified message string

logind: fix crash in logind on user-specified message string

This is trivially exploitable (in the sense of causing a crash from SEGV) e.g.
by 'shutdown now "Message %s %s %n"'. The message is settable through polkit,
but is limited to auth_admin:

<action id="org.freedesktop.login1.set-wall-message">
         <description gettext-domain="systemd">Set a wall message</description>
         <message gettext-domain="systemd">Authentication is required to set a wall message</message>
         <defaults>
                <allow_any>auth_admin_keep</allow_any>
                <allow_inactive>auth_admin_keep</allow_inactive>
                <allow_active>auth_admin_keep</allow_active>
        </defaults>
</action>

Bug introduced in 9ef15026c0e7e6600372056c43442c99ec53746e
('logind/systemctl: introduce SetWallMessage and --message', 2015-09-15).