dnsdist: Fix the oustanding counter when an exception is raised

dnsdist: Fix the oustanding counter when an exception is raised

If an exception is raised in the responder thread after the outstanding
queries counter has been decremented, but before we could mark the
state as processed, the same state would be processed again and the
counter decremented a second time, either because of a duplicate
answer or more likely by the timeout handler.
This commit simply increase back the outstanding counter when such
an exception occurs.