more defensive coding: guard against a non-iterable loclist (#4912)

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>

autoload/ale/engine.vim

@@ -178,6 +178,12 @@ function! s:HandleExit(job_info, buffer, output, data) abort
         let l:loclist = []
     endtry
 
+    if type(l:loclist) isnot# v:t_list
+        " we only expect the list type; don't pass anything else down to
+        " `ale#engine#HandleLoclist` since it won't understand it
+        let l:loclist = []
+    endif
+
     call ale#engine#HandleLoclist(l:linter.name, a:buffer, l:loclist, 0)
 endfunction