patch 8.1.0897: can modify a:000 when using a reference

Problem:    Can modify a:000 when using a reference.
Solution:   Make check for locked variable stricter. (Ozaki Kiichi,
            closes #3930)
diff --git a/src/testdir/test_let.vim b/src/testdir/test_let.vim
index 550655c..40eabb8 100644
--- a/src/testdir/test_let.vim
+++ b/src/testdir/test_let.vim
@@ -142,7 +142,7 @@
   call assert_fails('call s:set_varg4(1)', 'E742:')
   call s:set_varg5([0])
   call assert_fails('call s:set_varg6(1)', 'E742:')
-  " call assert_fails('call s:set_varg7(1)', 'E46:')
+  call assert_fails('call s:set_varg7(1)', 'E742:')
   call assert_fails('call s:set_varg8(1)', 'E742:')
   call s:set_varg9([0])
 endfunction