[Reviewer Andy] Handle pjsip_tsx_send_msg failures correctly

Andy

Can you review my fix to issue #782.  I'm pretty sure this fixes the particular crash that was hit as the call stack was pretty definitive, but as per the issue notes I haven't been able to reproduce the problem so I haven't been able to verify the fix by testing.  The UTs do run cleanly.  I did look at trying to reproduce in a UT, but it would require the ability to have the fake transport immediately fail a request to create a transport, which we don't currently have any way to do.

Mike