patch 9.0.1611: v:maxcol can be changed in a :for loop

Problem:    v:maxcol can be changed in a :for loop.
Solution:   Check for read-only loop variable. (closes #12470)
diff --git a/src/testdir/test_eval_stuff.vim b/src/testdir/test_eval_stuff.vim
index 46ec12d..a346399 100644
--- a/src/testdir/test_eval_stuff.vim
+++ b/src/testdir/test_eval_stuff.vim
Binary files differ