[BUG #8020] protect self.curr_hint.search_upward()

- when CreateHintsVisitor is instantiated process_comments() is run *before*
  self.curr_hint is initialized
- need to guard against this case
- (i wouldn't want to refac the entire process_comments interface)
- (the real culprit is the fact that process_comments is forced to print
  warnings from the comment parsing, s. comment in jshints.py)