Escape the filename for finddir

autoload/ale/path.vim

@@ -46,6 +46,7 @@ endfunction
 " through the paths relative to the given buffer.
 function! ale#path#FindNearestDirectory(buffer, directory_name) abort
     let l:buffer_filename = fnamemodify(bufname(a:buffer), ':p')
+    let l:buffer_filename = fnameescape(l:buffer_filename)
 
     let l:relative_path = finddir(a:directory_name, l:buffer_filename . ';')