Ensure that a non-zero sample rate is returned for all formats.

Ensure that a non-zero sample rate is returned for all formats.

Versions of Asterisk prior to 12 defaulted to 8000 as a sample rate
if one was not provided by a format. In Asterisk 13, this was removed.
The result was that some calculations which involve dividing by the
sample rate resulted in dividing by 0. The fix being put in place
here is to have the same default fallback that was present in previous
versions of Asterisk.

Asterisk-24914 #close
Reported by Marcello Ceschia
........

Merged revisions 434046 from http://svn.asterisk.org/svn/asterisk/branches/13

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@434047 65c4cc65-6c06-0410-ace0-fbb531ad65f3