Prevent crash on syntax error, from ksh93 repo.

bin/osh -n ~/git/other/ast/src/cmd/ksh93/tests/expand.sh
Line 119 of '/home/andy/git/other/ast/src/cmd/ksh93/tests/expand.sh'
  for i in ~(N)/dev/non_existent_file
            ^
Invalid word in for loop

osh/cmd_parse.py

@@ -23,6 +23,7 @@

log = util.log
command_e = ast.command_e
word_e = ast.word_e
assign_op = ast.assign_op


@@ -720,6 +721,10 @@ def ParseForWords(self):
      elif self.c_id == Id.Op_Newline:
        self._Next()
        break
      if self.cur_word.tag != word_e.CompoundWord:
        self.AddErrorContext('Invalid word in for loop', word=self.cur_word)
        return None

      words.append(self.cur_word)
      self._Next()
    return words, semi_spid