patch 8.2.3886: can define autocmd for every event by using "au!"

Problem:    Can define autocmd for every event by using "au!".
Solution:   Check if a command is present also for "au!".
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index 4bdb8dc..b6f621d 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
Binary files differ