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=7fc2c442c85aefe21e61502f50b27651b043b5cd'>root</a>/<a href='/guix/log/gnu?id=7fc2c442c85aefe21e61502f50b27651b043b5cd'>gnu</a>/<a href='/guix/log/gnu/services?id=7fc2c442c85aefe21e61502f50b27651b043b5cd'>services</a>/<a href='/guix/log/gnu/services/herd.scm?id=7fc2c442c85aefe21e61502f50b27651b043b5cd'>herd.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/services/herd.scm?id=7fc2c442c85aefe21e61502f50b27651b043b5cd&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-04-21 16:16:37 +0200'>2023-04-21</span></td><td><a href='/guix/commit/gnu/services/herd.scm?id=95731b4ef1991f5b85aa9ec9d4fb97c006bec65d'>services: herd: 'load-services/safe' is synonymous with 'load-services'.</a><span class='msg-avail'>...<span class='msg-tooltip'>This is a followup to 547965aa27b6a09cadf42130b7ec7db3f1aee61f.

* gnu/services/herd.scm (load-services/safe): Make an alias for
'load-services'.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2023-03-26 23:24:23 +0200'>2023-03-26</span></td><td><a href='/guix/commit/gnu/services/herd.scm?id=547965aa27b6a09cadf42130b7ec7db3f1aee61f'>services: herd: Remove workaround for Shepherd &lt; 0.5.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/herd.scm (load-services/safe): Remove workaround for
Shepherd &lt; 0.5.0, released in 2018.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-05-28 23:23:36 +0200'>2022-05-28</span></td><td><a href='/guix/commit/gnu/services/herd.scm?id=b04ae71defe0bd2a5fbd2df07d55cfe3eb40cba9'>services: herd: Add 'wait-for-service'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/herd.scm (wait-for-service): New procedure.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-04-10 00:17:40 +0200'>2022-04-10</span></td><td><a href='/guix/commit/gnu/services/herd.scm?id=a2c759c8304c461d096ab763568e7f71546ff4e8'>services: herd: Report whether a service is transient.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/herd.scm (&lt;live-service&gt;)[transient?]: New field.
(current-services): Check the value of 'transient?'.  Call
'resolve-transients'.
(resolve-transients): New procedure.
</span></span></td><td>Ludovic Courtès</td></tr>