funcs/func_cdr: Fix non-epoch timestamps broken by improper char array deref

Thanks to snuffy for pointing this issue out and fixing it.

(closes issue ASTERISK-23250)
Reported by: snuffy
patches:
  func_cdr-fix.diff uploaded by snuffy (License 5024)
........

Merged revisions 407259 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@407260 f38db490-d61c-443f-a65b-d21fe96a405b