patch 8.0.1413: accessing freed memory in :cbuffer

Problem:    Accessing freed memory in :cbuffer.
Solution:   Get quickfix list after executing autocmds. (closes #2470)
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index d8fddff..0636c47 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -1178,3 +1178,10 @@
   call assert_fails('lv½ /x', 'E480')
   au!
 endfunc
+
+func Test_wipe_cbuffer()
+  sv x
+  au * * bw
+  lb
+  au!
+endfunc