patch 9.0.0805: filetype autocmd may cause freed memory access

Problem:    Filetype autocmd may cause freed memory access.
Solution:   Set the quickfix-busy flag while filling the buffer.
diff --git a/src/testdir/test_quickfix.vim b/src/testdir/test_quickfix.vim
index 2ee754b..bcaef5d 100644
--- a/src/testdir/test_quickfix.vim
+++ b/src/testdir/test_quickfix.vim
@@ -3455,6 +3455,21 @@
   endtry
 endfunc
 
+func Test_filetype_autocmd()
+  " this changes the location list while it is in use to fill a buffer
+  lexpr ''
+  lopen
+  augroup FT_loclist
+    au FileType * call setloclist(0, [], 'f')
+  augroup END
+  silent! lolder
+  lexpr ''
+
+  augroup FT_loclist
+    au! FileType
+  augroup END
+endfunc
+
 func Test_vimgrep_with_textlock()
   new
 
@@ -6372,4 +6387,5 @@
   call setqflist([], 'f')
 endfunc
 
+
 " vim: shiftwidth=2 sts=2 expandtab