patch 8.0.0859: NULL pointer access when term_free_vterm called twice

Problem:    NULL pointer access when term_free_vterm called twice.
Solution:   Return when tl_vterm is NULL. (Yasuhiro Matsumoto, closes #1934)
diff --git a/src/version.c b/src/version.c
index e50eeaf..a5e02a1 100644
--- a/src/version.c
+++ b/src/version.c
@@ -770,6 +770,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    859,
+/**/
     858,
 /**/
     857,