soup-session: fix race in soup_session_real_kick_queue()

Jakub Janků requested to merge jjanku/libsoup:fix-kick-queue into master

I was hoping for some discussion on the related merge request page where I commented, but this wasn't the case, so I'm now submitting my own merge request with the proposed fix here.

Related: !37 (closed), #100 (closed)

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