Fix arithmetic parse error, and PS4 ParseError handling.

The arithmetic error was caused by ; being miscategorized.  It would
become part of an invalid arithmetic expression tree, and then the
runtime would crash.

The PS4 error was introduced in previous commits.

core/cmd_exec.py

@@ -1482,9 +1482,17 @@ def _EvalPS4(self):
      # NOTE: Reading PS4 is just like reading a here doc line.  "\n" is
      # allowed too.  The OUTER mode would stop at spaces, and ReadWord
      # doesn't allow lex_mode_e.DQ.
      ps4_word = w_parser.ReadHereDocBody()
      ok = True
      try:
        ps4_word = w_parser.ReadHereDocBody()
      except util.ParseError as e:
        ok = False
      else:
        # TODO: Get rid of duplicate forms of error handling.
        if not ps4_word:
          ok = False

      if not ps4_word:
      if not ok:
        error_str = '<ERROR: cannot parse PS4>'
        t = ast.token(Id.Lit_Chars, error_str, const.NO_INTEGER)
        ps4_word = ast.CompoundWord([ast.LiteralPart(t)])

osh/arith_parse.py

@@ -122,14 +122,14 @@ def MakeShellSpec():
  # -1 precedence -- doesn't matter
  spec.Null(-1, tdop.NullConstant, [
      Id.Word_Compound,
      Id.Arith_Semi,  # for loop
  ])
  spec.Null(-1, tdop.NullError, [
      Id.Arith_RParen, Id.Arith_RBracket, Id.Arith_Colon,
      Id.Eof_Real, Id.Eof_RParen, Id.Eof_Backtick,
      # Not in the arithmetic language, but is a common terminator, e.g.
      # ${foo:1}
      Id.Arith_RBrace,

      # Not in the arithmetic language, but useful to define here.
      Id.Arith_Semi,  # terminates loops like for (( i = 0 ; ... ))
      Id.Arith_RBrace,  # terminates slices like ${foo:1}
  ])

  # 0 precedence -- doesn't bind until )

osh/arith_parse_test.py

@@ -159,6 +159,10 @@ def testErrors(self):
    testSyntaxError('( 1')
    testSyntaxError('(1 + (3 * 4)')
    testSyntaxError('(1 + (3 * 4) 5')  # Not valid, expr also fails.

    testSyntaxError(';')
    testSyntaxError('- ;')

    #testSyntaxError('1 1')
    #testSyntaxError('( 1 ) ( 2 )')