Fix false reporting of an error by set_format().

Fix false reporting of an error by set_format().

In the case that the native format was able to be changed to match the
new requested format, the code proceeded to attempt to build a translation
path, anyway.  The result would be NULL, since no translation path is
necessary and resulted in this function thinking an error has occurred.
This case is now specifically caught and no attempt to build a translation
path is attempted.

Thanks to our automated tests and bamboo.asterisk.org for catching this problem
and making a whole lot of noise when things started failing.  :-)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.4@296082 65c4cc65-6c06-0410-ace0-fbb531ad65f3