Fix CVE-2017-5953:

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-5953
https://groups.google.com/forum/#!topic/vim_dev/t-3RSdEnrHY

Patch adapted from upstream source repository:

https://github.com/vim/vim/commit/399c297aa93afe2c0a39e2a1b3f972aebba44c9d

From 399c297aa93afe2c0a39e2a1b3f972aebba44c9d Mon Sep 17 00:00:00 2001
From: Bram Moolenaar <Bram@vim.org>
Date: Thu, 9 Feb 2017 21:07:12 +0100
Subject: [PATCH] patch 8.0.0322: possible overflow with corrupted spell file

Problem:    Possible overflow with spell file where the tree length is
            corrupted.
Solution:   Check for an invalid length (suggested by shqking)
---
 src/spellfile.c | 3 +++
 src/version.c   | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/src/spellfile.c b/src/spellfile.c
index c7d87c6..8b1a3a6 100644
--- a/src/spellfile.c
+++ b/src/spellfile.c
@@ -1595,6 +1595,9 @@ spell_read_tree(
     len = get4c(fd);
     if (len < 0)
 	return SP_TRUNCERROR;
+    if (len >= 0x3ffffff)
+	/* Invalid length, multiply with sizeof(int) would overflow. */
+	return SP_FORMERROR;
     if (len > 0)
     {
 	/* Allocate the byte array. */