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=b331a07a469faf0aeb7bc11f42399b9e986e8e8d'>root</a>/<a href='/guix/log/m4?id=b331a07a469faf0aeb7bc11f42399b9e986e8e8d'>m4</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/m4?id=b331a07a469faf0aeb7bc11f42399b9e986e8e8d&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2024-12-03 08:38:00 +0100'>2024-12-03</span></td><td><a href='/guix/commit/m4?id=4d9c5984fee481d74c2f504094b4797bbb4104d4'>gnu: bootstrap: Add support for x86_64-gnu, aka the 64bit Hurd.</a><span class='msg-avail'>...<span class='msg-tooltip'>On commit:
    ec8a5ec15f898e864705e5a5c834532e3fa8d0a4
    gnu: make-bootstrap: Update gcc-static to gcc-14, for the 64bit Hurd.

Run:
    ./pre-inst-env guix build --target=x86_64-gnu bootstrap-tarballs

Producing:
    /gnu/store/w1n7bdpn88plcc49h7n0jriaj41sgwx8-bootstrap-tarballs-0/

With guix hash -r:
    15cb1xh7s2hhp8s0d81bjnw1759w9sh7ckc9n5jq2f3rqw6z76by

* gnu/packages/bootstrap.scm (%bootstrap-executables): Add entries for
x86_64-gnu.
(%bootstrap-guile-hash, %bootstrap-coreutils&amp;co, %bootstrap-binutils,
%bootstrap-glibc, %bootstrap-gcc): Add entry for x86_64-gnu.
* guix/packages.scm (%supported-systems, %hurd-systems): Add x86_64-gnu.
(%cuirass-supported-systems): Remove x86_64-gnu.
* guix/utils.scm (target-64bit?): Add x86_64-gnu.
* m4/guix.m4: Add x86_64-gnu as a supported system.
* doc/guix.texi (GNU Distribution): Add x86_64-gnu.

Change-Id: I828159aedb3f66caba98e935083cc3682429f219
</span></span></td><td>Janneke Nieuwenhuizen</td></tr>
<tr><td><span title='2023-11-07 18:07:11 +0200'>2023-11-07</span></td><td><a href='/guix/commit/m4?id=220759226e93d76d8d80058f69f9d8b29714bbde'>build: Fix spelling in m4 macros.</a><span class='msg-avail'>...<span class='msg-tooltip'>* m4/guix.m4 (GUIX_CHECK_GUILE_SSH): Fix spelling of parameter.
(GUIX_CHECK_FILE_NAME_LIMITS): Fix spelling of maximum.

Change-Id: I93c80441393622a4bc06daa475eee13874cca527
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2023-03-16 12:37:03 +0100'>2023-03-16</span></td><td><a href='/guix/commit/m4?id=8e3dfb3d6dfc0f19bd9a9f9b163f0f3d7dccc865'>build: Correct guix_system on musl libc distros.</a><span class='msg-avail'>...<span class='msg-tooltip'>* m4/guix.m4 (GUIX_SYSTEM_TYPE): Add linux-musl* case. This prevents
the macro from mis-parsing the host OS, which causes breakage when
building from source.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Antero Mejr</td></tr>