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