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/ex_docmd.c b/src/ex_docmd.c
index 6df6975..80cc930 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -6572,7 +6572,7 @@
     int		dir_differs;
     int		retval = FALSE;
 
-    if (allbuf_locked())
+    if (new_dir == NULL || allbuf_locked())
 	return FALSE;
 
     if (vim_strchr(p_cpo, CPO_CHDIR) != NULL && curbufIsChanged() && !forceit)