Sometimes `s:HandleExit` can execute a deferred linter callback, which
ends up setting the `l:loclist` that's passed into
`ale#engine#HandleLoclist` at the end of `s:HandleExit` to a dictionary.
This dictionary cannot be iterated over, and thus errors out.
Guard against trying to iterate over values that don't make sense.
Co-authored-by: Alexander Huynh <git@e.sc>
