Add a missing error message, silencing stack trace.

osh/arith_parse.py

@@ -18,6 +18,8 @@ def NullIncDec(p, w, bp):
  """ ++x or ++x[1] """
  right = p.ParseUntil(bp)
  child = tdop.ToLValue(right)
  if child is None:
    p_die("This value can't be assigned to", word=w)
  return ast.UnaryAssign(word.ArithId(w), child)