patch 8.0.1403: using freed buffer in grep command

Problem:    Using freed buffer in grep command. (gy741, Dominique Pelle)
Solution:   Lock the dummy buffer to avoid autocommands wiping it out.
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 8ac8811..d8fddff 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -1171,3 +1171,10 @@
   bwipe
   au!
 endfunc
+
+func Test_nocatch_wipe_dummy_buffer()
+  " Nasty autocommand: wipe buffer on any event.
+  au * x bwipe
+  call assert_fails('lv½ /x', 'E480')
+  au!
+endfunc