TaskEventTest: fix possible null object deref

Probably found by coverity? Can't recall if I actually had the bug,
but might as well fix it!

Change-Id: I10680dbd38eb54803223798ff15334d910f5cbd9

Author: Dominique Martinet

tests/TaskEventTest.py

@@ -238,8 +238,8 @@ def testEngineMayReuseFD(self):
             eh = AbortOnReadHandler()
             for i in range(10):
                 worker = task.shell("echo ok; sleep 1", handler=eh)
-                worker.write("OK\n")
                 self.assert_(worker is not None)
+                worker.write("OK\n")
             task.resume()
         finally:
             task.set_info("fanout", fanout)