Fix udptl issue with initial INVITE introduced by r351027

Fix udptl issue with initial INVITE introduced by r351027

When an inital INVITE occurs that contains image media, a channel
is not yet associated with the SIP dialog.  The file descriptor
associated with the udptl session needs to be set in
initialize_udptl or in sip_new to account for this scenario.
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Merged revisions 351233 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 351234 from http://svn.asterisk.org/svn/asterisk/branches/10

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@351235 65c4cc65-6c06-0410-ace0-fbb531ad65f3