Replace magic number size from msgArray array with a define.

(same patch as before, I just split this part out)
(close issue #12326)
Reported by: travishein
Patches:
      app_voicemail_code_documentation.patch uploaded by travishein (license 385)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@111774 65c4cc65-6c06-0410-ace0-fbb531ad65f3