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=6ed5b496bc1a2199fe5d40b5186df38789ff6937'>root</a>/<a href='/guix/log/tests?id=6ed5b496bc1a2199fe5d40b5186df38789ff6937'>tests</a>/<a href='/guix/log/tests/cache.scm?id=6ed5b496bc1a2199fe5d40b5186df38789ff6937'>cache.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/tests/cache.scm?id=6ed5b496bc1a2199fe5d40b5186df38789ff6937&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-06-04 12:04:23 +0200'>2022-06-04</span></td><td><a href='/guix/commit/tests/cache.scm?id=104b4e25ab7da5697f2f6f1ddfdd4955f05afece'>cache: Catch invalid 'last-expiry-cleanup'.</a><span class='msg-avail'>...</span></td><td>zimoun</td></tr>
<tr><td><span title='2020-03-22 16:49:47 +0100'>2020-03-22</span></td><td><a href='/guix/commit/tests/cache.scm?id=9acacb71c958218fd69cf0fb9df0b439a980a0f2'>Remove workaround for 'time-monotonic' in Guile 2.2.2.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
2:07 +0100'>2020-01-17</span></td><td><a href='/guix/commit/tests/opam.scm?id=282f91790a5bbd0d9b3223c4bfd45b66e418c200'>import: opam: Avoid uses of '@@' in tests.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-09-07 22:25:14 +0200'>2019-09-07</span></td><td><a href='/guix/commit/tests/opam.scm?id=1d03a9198db6f3656a34d62eb89e5f7d5a99e76a'>tests: opam: Fix input type in import test.</a><span class='msg-avail'>...</span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2019-02-05 22:33:06 +0100'>2019-02-05</span></td><td><a href='/guix/commit/tests/opam.scm?id=0f4432c620f8d569ef29ca74bad2b7eebd803441'>import: opam: Fix conditions.</a><span class='msg-avail'>...</span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2018-12-17 21:43:51 +0100'>2018-12-17</span></td><td><a href='/guix/commit/tests/opam.scm?id=cce654fabdf09cac7d18f9bad842ba8445aa022c'>import: Update opam importer.</a><span class='msg-avail'>...</span></td><td>Julien Lepiller</td></tr>
<tr><td><span title='2018-09-04 17:25:11 +0200'>2018-09-04</span></td><td><a href='/guix/commit/tests/opam.scm?id=ca719424455465fca4b872c371daf2a46de88b33'>Switch to Guile-Gcrypt.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-07-10 10:11:01 +0200'>2018-07-10</span></td><td><a href='/guix/commit/tests/opam.scm?id=b24443bff9f9f3f36353eea2ef35e6dc3745a417'>guix: Add opam importer.</a><span class='msg-avail'>...</span></td><td>Julien Lepiller</td></tr>