[word_eval] Fix the ${!prefix@} failure for the second time

[akinomyoga]

See the fourth item in #653 (comment).

See the following test case. Even though it succeeds for the first time, it fails for the second time.

test3.sh

#!/usr/bin/env bash

abc_hello=1
function hello1 { echo "${!abc_@}"; }
hello1
hello1

$ osh test3.sh
abc_hello
  function hello1 { echo "${!abc_@}"; }
                             ^~~~
test3.sh:4: fatal: Bad indirect expansion: ''

[andychu]

Wow this is embarrassing -- the perils of test-driven development with narrow tests! :-/

I remember I filed #604 at this time because I found the code confusing.

Note to self: don't mutate the AST! Thank you.