When ending a recording with silence detection, remember to reduce the duration.

When ending a recording with silence detection, remember to reduce the duration.
The end of the recording is correspondingly trimmed, but the duration was not
trimmed by the number of seconds trimmed, so the saved duration was necessarily
longer than the actual soundfile duration.
(closes issue #14406)
 Reported by: sasargen
 Patches:
       20090226__bug14406.diff.txt uploaded by tilghman (license 14)
 Tested by: sasargen

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@179468 65c4cc65-6c06-0410-ace0-fbb531ad65f3