patch 8.1.0205: invalid memory access with invalid modeline

Problem:    Invalid memory access with invalid modeline.
Solution:   Pass pointer limit. Add a test. (closes #3241)
diff --git a/src/testdir/test_alot.vim b/src/testdir/test_alot.vim
index ac3ca74..1465a7a 100644
--- a/src/testdir/test_alot.vim
+++ b/src/testdir/test_alot.vim
@@ -37,6 +37,7 @@
 source test_match.vim
 source test_menu.vim
 source test_messages.vim
+source test_modeline.vim
 source test_partial.vim
 source test_popup.vim
 source test_put.vim