patch 8.0.0703: illegal memory access with empty :doau command

Problem:    Illegal memory access with empty :doau command.
Solution:   Check the event for being out of range. (James McCoy)
diff --git a/src/testdir/test_autocmd.vim b/src/testdir/test_autocmd.vim
index ef28034..54a2b3f 100644
--- a/src/testdir/test_autocmd.vim
+++ b/src/testdir/test_autocmd.vim
@@ -418,3 +418,7 @@
     call delete(file)
   endfor
 endfunc
+
+func Test_empty_doau()
+  doau \|
+endfunc