Fix referenced before assignment for BuildCompletedEvent

It was possible for duration to be referenced before assignment.

  Traceback (most recent call last):
    File "/usr/local/lib/python2.7/dist-packages/zuul/scheduler.py", line 1108, in _doBuildCompletedEvent
      except Exception:
  UnboundLocalError: local variable 'duration' referenced before assignment

Change-Id: I48446f9cdb23bfd37c1a843dbcf90f94492a0678
Signed-off-by: Paul Belanger <pabelanger@redhat.com>
1 file changed