Make sure that the temporary sip_request structure is empty so that...

Make sure that the temporary sip_request structure is empty so that copy_request doesn't think it already has an ast_str.
(closes issue #12231)
Reported by: IgorG


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@109054 65c4cc65-6c06-0410-ace0-fbb531ad65f3