patch 9.0.0404: crash when passing invalid arguments to assert_fails()

Problem:    Crash when passing invalid arguments to assert_fails().
Solution:   Check for NULL string.
diff --git a/src/version.c b/src/version.c
index db1a1de..36ce07e 100644
--- a/src/version.c
+++ b/src/version.c
@@ -704,6 +704,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    404,
+/**/
     403,
 /**/
     402,