patch 8.0.0331: restoring help snapshot accesses freed memory

Problem:    Restoring help snapshot accesses freed memory. (Dominique Pelle)
Solution:   Don't restore a snapshot when the window closes.
diff --git a/src/testdir/test_help.vim b/src/testdir/test_help.vim
new file mode 100644
index 0000000..ca095d0
--- /dev/null
+++ b/src/testdir/test_help.vim
@@ -0,0 +1,10 @@
+" Tests for :help
+
+func Test_help_restore_snapshot()
+  help
+  set buftype=
+  help
+  edit x
+  help
+  helpclose
+endfunc