patch 9.0.0691: lalloc(0) error in listchars test

Problem:    lalloc(0) error in listchars test.
Solution:   Skip generating text for tab if tab_len is zero.
diff --git a/src/version.c b/src/version.c
index 37046ba..11fa0ce 100644
--- a/src/version.c
+++ b/src/version.c
@@ -700,6 +700,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    691,
+/**/
     690,
 /**/
     689,