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/terminal.c b/src/terminal.c
index a4120b3..893cd0f 100644
--- a/src/terminal.c
+++ b/src/terminal.c
@@ -691,8 +691,11 @@
     VTermPos	    pos;
     VTermScreenCell cell;
     VTermScreenCell *p;
-    VTermScreen	    *screen = vterm_obtain_screen(term->tl_vterm);
+    VTermScreen	    *screen;
 
+    if (term->tl_vterm == NULL)
+	return;
+    screen = vterm_obtain_screen(term->tl_vterm);
     for (pos.row = 0; pos.row < term->tl_rows; ++pos.row)
     {
 	len = 0;
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,