Fix bugs with directory listings and signs when switching tabs.

plugin/ale/cursor.vim

@@ -53,6 +53,10 @@ function! ale#cursor#TruncatedEcho(message)
 endfunction
 
 function! ale#cursor#EchoCursorWarning()
+    if !exists('b:ale_loclist')
+        return
+    endif
+
     let pos = getcurpos()
 
     let index = s:BinarySearch(b:ale_loclist, pos[1], pos[2])

plugin/ale/sign.vim

@@ -26,7 +26,9 @@ sign define ALEWarningSign text=-- texthl=ALEWarningSign
 
 " This function will set the signs which show up on the left.
 function! ale#sign#SetSigns(loclist)
-    sign unplace *
+    let buffer = bufnr('%')
+
+    exec 'sign unplace * buffer=' . buffer
 
     for i in range(0, len(a:loclist) - 1)
         let obj = a:loclist[i]