patch 8.1.0404: accessing invalid memory with long argument name

Problem:    Accessing invalid memory with long argument name.
Solution:   Use item_count instead of checking for a terminating NULL.
            (Dominique Pelle, closes #3444)
diff --git a/src/testdir/test_arglist.vim b/src/testdir/test_arglist.vim
index a6c71c9..c558aab 100644
--- a/src/testdir/test_arglist.vim
+++ b/src/testdir/test_arglist.vim
@@ -411,3 +411,10 @@
   call assert_equal('notexist Xx\ x runtest.vim', expand('##'))
   call delete('Xx x')
 endfunc
+
+func Test_large_arg()
+  " Argument longer or equal to the number of columns used to cause
+  " access to invalid memory.
+  exe 'argadd ' .repeat('x', &columns)
+  args
+endfunc