When mod_auth_radius authenticates user against remote RADIUS server,
it will send RADIUS packet with RADIUS_ACCESS_REQUEST code.
Server can respond with RADIUS packet with RADIUS_ACCESS_CHALLENGE code.
When mod_auth_radius gets RADIUS_ACCESS_CHALLENGE with attribute code set
to RADIUS_STATE and another attribute code in same packet set to RADIUS_REPLY_MESSAGE,
RADIUS server reply will be copied in the local buffer with function radcpy().
Size of the data to be copied in local buffer is taken from 'length' value of the packet
attribute received from RADIUS server.