Simplify code by using the -= and %= operators.

(closes issue #10848)
Reported by: opticron
Patches: 
      saymod.diff uploaded by opticron (license 267)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@84172 65c4cc65-6c06-0410-ace0-fbb531ad65f3