runner.gevent: Shift None check for topic_func earlier

Avoid:

  Traceback (most recent call last):
    File "/.../pyvows/runner/gevent.py", line 97, in _run_setup_and_topic
      topic_list = get_topics_for(topic_func, ctx_obj)
    File "/.../pyvows/runner/utils.py", line 46, in get_topics_for
      'Function %s does not have a code property' % topic_function)
  RuntimeError: Function None does not have a code property

because VowsParallelRunner._run_setup_and_topic was calling
get_topics_for (which chokes when it's topic_function is None) before
checking for a None value.

The fixes a broken check from 59e67374 (topics can be none, we need to
validate against that, 2014-06-02).