Previously, FILE() returned one less character than specified, due to the
terminating NULL. Both the offset and length parameters now behave
identically to the way variable substitution offsets and lengths also work.
(closes issue #14670)
Reported by: BMC
git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@182278 65c4cc65-6c06-0410-ace0-fbb531ad65f3