If imapfolder=foo were set in voicemail.conf, then when calling VoiceMailMain,

app_voicemail would attempt to play a file called vm-foo instead of playing
vm-INBOX to play the "new" sound file. This commit fixes that issue.

This may fix one of the problems reported in issue #12987



git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@128731 65c4cc65-6c06-0410-ace0-fbb531ad65f3