Copied from Debian.

From c18f27a58792f7fbd0b30a0ff50d6b40a82f940d Mon Sep 17 00:00:00 2001
From: Geoff McLane <ubuntu@geoffair.info>
Date: Wed, 3 Jun 2015 20:26:03 +0200
Subject: [PATCH] Issue #217 - avoid len going negative, ever...

---
 src/lexer.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/src/lexer.c b/src/lexer.c
index 376a3d8..664f806 100644
--- a/src/lexer.c
+++ b/src/lexer.c
@@ -3739,16 +3740,17 @@ static tmbstr ParseValue( TidyDocImpl* doc, ctmbstr name,
         /* and prompts attributes unless --literal-attributes is set to yes      */
         /* #994841 - Whitespace is removed from value attributes                 */
 
-        if (munge &&
+        /* Issue #217 - Also only if/while (len > 0) - MUST NEVER GO NEGATIVE! */
+        if ((len > 0) && munge &&
             TY_(tmbstrcasecmp)(name, "alt") &&
             TY_(tmbstrcasecmp)(name, "title") &&
             TY_(tmbstrcasecmp)(name, "value") &&
             TY_(tmbstrcasecmp)(name, "prompt"))
         {
-            while (TY_(IsWhite)(lexer->lexbuf[start+len-1]))
+            while (TY_(IsWhite)(lexer->lexbuf[start+len-1]) && (len > 0))
                 --len;
 
-            while (TY_(IsWhite)(lexer->lexbuf[start]) && start < len)
+            while (TY_(IsWhite)(lexer->lexbuf[start]) && (start < len) && (len > 0))
             {
                 ++start;
                 --len;
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<div class='path'>path: <a href='/guix/log/?id=82c8e0ae0a81aedc65593d630b9ee4931a1e6a8d'>root</a>/<a href='/guix/log/srfi?id=82c8e0ae0a81aedc65593d630b9ee4931a1e6a8d'>srfi</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/srfi?id=82c8e0ae0a81aedc65593d630b9ee4931a1e6a8d&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2017-06-30 00:16:50 +0200'>2017-06-30</span></td><td><a href='/guix/commit/srfi?id=1d97fd8cb60f0b5f386f834940f859ff0b0bcc71'>build: Remove check for broken (srfi srfi-37).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2014-02-24 16:30:08 -0500'>2014-02-24</span></td><td><a href='/guix/commit/srfi?id=8fade76f5d35fac321a5a2c4c6e142f03a69fcfa'>Update SRFI-64 to the latest upstream version.</a><span class='msg-avail'>...</span></td><td>Mark H Weaver</td></tr>