config: Fix CB_ADD_LEN() to work as originally intended.

Fix a long standing bug in CB_ADD_LEN() behaving like CB_ADD().

ASTERISK-23546 #close
Reported by: Walter Doekes


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@411960 f38db490-d61c-443f-a65b-d21fe96a405b