patch 8.2.0381: using freed memory with :lvimgrep and autocommand

Problem:    Using freed memory with :lvimgrep and autocommand. (extracted from
            POC by Dominique Pelle)
Solution:   Avoid deleting a dummy buffer used in a window. (closes #5777)
diff --git a/src/testdir/test_quickfix.vim b/src/testdir/test_quickfix.vim
index 2b51b57..c1b5063 100644
--- a/src/testdir/test_quickfix.vim
+++ b/src/testdir/test_quickfix.vim
@@ -3684,6 +3684,14 @@
   enew | only
 endfunc
 
+func Test_lvimgrep_crash2()
+  au BufNewFile x sfind
+  call assert_fails('lvimgrep x x', 'E480:')
+  call assert_fails('lvimgrep x x x', 'E480:')
+
+  au! BufNewFile
+endfunc
+
 " Test for the position of the quickfix and location list window
 func Test_qfwin_pos()
   " Open two windows