patch 8.2.0189: cd() with NULL argument crashes

Problem:    cd() with NULL argument crashes.
Solution:   Check for NULL. (Ken Takata, closes #5558)
diff --git a/src/version.c b/src/version.c
index 9a3d42f..5f090c3 100644
--- a/src/version.c
+++ b/src/version.c
@@ -743,6 +743,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    189,
+/**/
     188,
 /**/
     187,