Ignore voicemail messages that are just silence.

(closes issue #2264)
Reported by: pfn
Patches:
      silent-vm-1.6.2.txt uploaded by pfn (license 810)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@202570 65c4cc65-6c06-0410-ace0-fbb531ad65f3