patch 8.1.0564: setting v:errors to wrong type still possible

Problem:    Setting v:errors to wrong type still possible.
Solution:   Return after giving an error message. (Christian Brabandt)
diff --git a/src/testdir/test_eval_stuff.vim b/src/testdir/test_eval_stuff.vim
index bcf45cc..a1aa788 100644
--- a/src/testdir/test_eval_stuff.vim
+++ b/src/testdir/test_eval_stuff.vim
@@ -53,3 +53,13 @@
 	"\ and some more
   call assert_equal([5, 6], array)
 endfunc
+
+func Test_E963()
+  " These commands used to cause an internal error prior to vim 8.1.0563
+  let v_e = v:errors
+  let v_o = v:oldfiles
+  call assert_fails("let v:errors=''", 'E963:')
+  call assert_equal(v_e, v:errors)
+  call assert_fails("let v:oldfiles=''", 'E963:')
+  call assert_equal(v_o, v:oldfiles)
+endfunc