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;
ee0'>commit</a><a href='/guix/diff/gnu/packages/django.scm?id=1c088531cf0be8fe6706c4624a920fca2ab2fee0'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/packages/django.scm'>
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<div class='path'>path: <a href='/guix/log/?id=1c088531cf0be8fe6706c4624a920fca2ab2fee0'>root</a>/<a href='/guix/log/gnu?id=1c088531cf0be8fe6706c4624a920fca2ab2fee0'>gnu</a>/<a href='/guix/log/gnu/packages?id=1c088531cf0be8fe6706c4624a920fca2ab2fee0'>packages</a>/<a href='/guix/log/gnu/packages/django.scm?id=1c088531cf0be8fe6706c4624a920fca2ab2fee0'>django.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/gnu/packages/django.scm?id=1c088531cf0be8fe6706c4624a920fca2ab2fee0&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2024-06-30 22:03:33 -0500'>2024-06-30</span></td><td><a href='/guix/commit/gnu/packages/django.scm?id=474b583e7ab0e54947183060024fd4e7181cca28'>gnu: python-django-4.2: Simpler unittest skip.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/django.scm (python-django-4.2): Simpler unittest skip.

Signed-off-by: jgart &lt;jgart@dismail.de&gt;
</span></span></td><td>Nicolas Graves</td></tr>
<tr><td><span title='2024-06-30 22:03:32 -0500'>2024-06-30</span></td><td><a href='/guix/commit/gnu/packages/django.scm?id=bfa5b66b02ed80d9bd3c354dfffcc074e2af2320'>gnu: python-django-4.2: Use pyproject-build-system.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/django.scm (python-django-4.2):
[build-system] Use pyproject-build-system.

Signed-off-by: jgart &lt;jgart@dismail.de&gt;
</span></span></td><td>Nicolas Graves</td></tr>
<tr><td><span title='2024-06-30 22:03:31 -0500'>2024-06-30</span></td><td><a href='/guix/commit/gnu/packages/django.scm?id=1c21ecff8c69f247491673b8a762ef39a418af38'>gnu: python-django-4.2: Add --failfast to test-flags.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/django.scm (python-django-4.2): Add --failfast to test-flags.

Signed-off-by: jgart &lt;jgart@dismail.de&gt;
</span></span></td><td>Nicolas Graves</td></tr>
<tr><td><span title='2024-06-30 22:03:30 -0500'>2024-06-30</span></td><td><a href='/guix/commit/gnu/packages/django.scm?id=85725446d60192cabcac05efad629906e1965aeb'>gnu: python-django-4.2: Comment formatting.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/django.scm (python-django-4.2): Comment formatting.

Signed-off-by: jgart &lt;jgart@dismail.de&gt;
</span></span></td><td>Nicolas Graves</td></tr>
<tr><td><span title='2024-06-30 22:03:30 -0500'>2024-06-30</span></td><td><a href='/guix/commit/gnu/packages/django.scm?id=5035e6d06d3f67cd40c5c93fbeff22c0af5504a1'>gnu: python-django-4.2: Use test-flags argument.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/django.scm (python-django-4.2): Use test-flags argument.

Signed-off-by: jgart &lt;jgart@dismail.de&gt;
</span></span></td><td>Nicolas Graves</td></tr>