Fix an off-by-one error in the FILE() function, and extend FILE()'s length...

Fix an off-by-one error in the FILE() function, and extend FILE()'s length parameter to work like variable substitution.
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