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


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@411961 65c4cc65-6c06-0410-ace0-fbb531ad65f3