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

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
........

Merged revisions 411960 from http://svn.asterisk.org/svn/asterisk/branches/1.8
........

Merged revisions 411961 from http://svn.asterisk.org/svn/asterisk/branches/11
........

Merged revisions 411962 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@411963 65c4cc65-6c06-0410-ace0-fbb531ad65f3