patch 8.0.1589: error for setting 'modifiable' when resetting it

Problem:    Error for setting 'modifiable' when resetting it.
Solution:   Check if 'modifiable' was actually set.
diff --git a/src/version.c b/src/version.c
index 8bbd1e4..d21b8b1 100644
--- a/src/version.c
+++ b/src/version.c
@@ -767,6 +767,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1589,
+/**/
     1588,
 /**/
     1587,