When using externhost make sure the port gets set to the bindaddr port if one...

When using externhost make sure the port gets set to the bindaddr port if one was not specified in the externhost value itself.
(closes issue #13634)
Reported by: performer


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@164659 65c4cc65-6c06-0410-ace0-fbb531ad65f3