patch 9.0.1531: crash when register contents ends up being invalid

Problem:    Crash when register contents ends up being invalid.
Solution:   Check "y_array" is not NULL.
diff --git a/src/version.c b/src/version.c
index 310803c..7ee9f57 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1531,
+/**/
     1530,
 /**/
     1529,