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/evalvars.c b/src/evalvars.c
index bebee2a..35a1d20 100644
--- a/src/evalvars.c
+++ b/src/evalvars.c
@@ -3842,7 +3842,8 @@
 	    }
 
 	    if ((flags & ASSIGN_FOR_LOOP) == 0
-				 && var_check_permission(di, name) == FAIL)
+				 ? var_check_permission(di, name) == FAIL
+				 : var_check_ro(di->di_flags, name, FALSE))
 		goto failed;
 	}
 	else
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
diff --git a/src/version.c b/src/version.c
index a8f2104..2e3c240 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1611,
+/**/
     1610,
 /**/
     1609,