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;
986ecf020c39edbc82da28089ee1'>commit</a><a href='/guix/diff/etc/time-travel-manifest.scm?id=c541a491aa82986ecf020c39edbc82da28089ee1'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/etc/time-travel-manifest.scm'>
<input type='hidden' name='id' value='c541a491aa82986ecf020c39edbc82da28089ee1'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=c541a491aa82986ecf020c39edbc82da28089ee1'>root</a>/<a href='/guix/log/etc?id=c541a491aa82986ecf020c39edbc82da28089ee1'>etc</a>/<a href='/guix/log/etc/time-travel-manifest.scm?id=c541a491aa82986ecf020c39edbc82da28089ee1'>time-travel-manifest.scm</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/etc/time-travel-manifest.scm?id=c541a491aa82986ecf020c39edbc82da28089ee1&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-08-17 15:42:56 +0200'>2022-08-17</span></td><td><a href='/guix/commit/etc/time-travel-manifest.scm?id=845c841e21379e64a4215ee6d3d60da343e7fb93'>time-travel-manifest: Use a separate cache per system.</a><span class='msg-avail'>...<span class='msg-tooltip'>* etc/time-travel-manifest.scm (&lt;guix-instance-compiler&gt;): Use a separate
cache per system so that parallel system evaluations do no step on each
other's toes.
</span></span></td><td>Mathieu Othacehe</td></tr>
<tr><td><span title='2022-07-10 23:57:14 +0200'>2022-07-10</span></td><td><a href='/guix/commit/etc/time-travel-manifest.scm?id=5d0437ea8ce0147b47b9df866fa2413b48f71a1a'>time-travel-manifest: Use a separate Git checkout cache.</a><span class='msg-avail'>...<span class='msg-tooltip'>* etc/time-travel-manifest.scm (guix-instance-compiler): Parameterize
%REPOSITORY-CACHE-DIRECTORY.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-07-08 23:58:11 +0200'>2022-07-08</span></td><td><a href='/guix/commit/etc/time-travel-manifest.scm?id=e80f0cda96181d90604c44f69094f2499036b3f6'>etc: Add 'time-travel-manifest.scm'.</a><span class='msg-avail'>...<span class='msg-tooltip'>This manifest makes it easy to test travels from the current revision
back to the revision of a past Guix release.

Suggested by zimoun &lt;zimon.toutoune@gmail.com&gt;.

* etc/time-travel-manifest.scm: New file.
* Makefile.am (EXTRA_DIST): Add it.
</span></span></td><td>Ludovic Courtès</td></tr>