"Undefined variable" with slicing

With a file like the following:

``` vim
function! F()
  let s = '123'
  let foobar = 1
  echo s[foobar:foobar+1]
endfunction
```

vint will wrongly say:

> t.vim:4:10: Undefined variable: foobar:foobar (see :help E738)

The workaround is to use `s[(foobar):(foobar+1)]`, but both works in Vim.