${arr[0]=1} change variable to string rather than assigning cell

[andychu]

Thank you for the fix! But the fix c0f9c2f revealed another bug. See the following test case.

$ osh -c 'arr=(); echo ${#arr[@]}; : ${arr[0]=1}; echo ${#arr[@]}'
0
  arr=(); echo ${#arr[@]}; : ${arr[0]=1}; echo ${#arr[@]}
                                               ^~
[ -c flag ]:1: fatal: Can't index string with @

Originally posted by @akinomyoga in #653 (comment)

[andychu]

May relate to IR in #604 , e.g. sh_lhs_expr