patch 9.0.0220: invalid memory access with for loop over NULL string

Problem:    Invalid memory access with for loop over NULL string.
Solution:   Make sure mb_ptr2len() consistently returns zero for NUL.
diff --git a/src/testdir/test_eval_stuff.vim b/src/testdir/test_eval_stuff.vim
index c63082e..313d791 100644
--- a/src/testdir/test_eval_stuff.vim
+++ b/src/testdir/test_eval_stuff.vim
Binary files differ