Fixed problems with ast_cdr_serialize_variables().

* Fixed return value of ast_cdr_serialize_variables() on error.  It needs
to return 0 indicating no CDR variables found.

* Made ast_cdr_serialize_variables() check the return value of
cdr_object_format_property() and assert if nonzero.  A member of the
cdr_readonly_vars[] was not handled.

* Removed unused elements from cdr_readonly_vars[]: total_duration,
total_billsec, first_start, and first_answer.
........

Merged revisions 397900 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@397901 65c4cc65-6c06-0410-ace0-fbb531ad65f3