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=f085755bb8af01065f5b8a2dba1ff93993e615fb'>root</a>/<a href='/guix/log/gnu?id=f085755bb8af01065f5b8a2dba1ff93993e615fb'>gnu</a>/<a href='/guix/log/gnu/packages?id=f085755bb8af01065f5b8a2dba1ff93993e615fb'>packages</a>/<a href='/guix/log/gnu/packages/sdl.scm?id=f085755bb8af01065f5b8a2dba1ff93993e615fb'>sdl.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/sdl.scm?id=f085755bb8af01065f5b8a2dba1ff93993e615fb&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-10-23 14:40:32 +0200'>2022-10-23</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=083e6b209b8e5a3b54966efcf6f4142f3e9ac548'>gnu: guile-sdl2: Update to 0.8.0.</a><span class='msg-avail'>...</span></td><td>Liliana Marie Prikler</td></tr>
<tr><td><span title='2022-10-04 20:40:46 +0200'>2022-10-04</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=c264265abd7f0aa8d62c301dc22bc72cc2055b0e'>gnu: sdl2: Use ibus-minimal instead of ibus.</a><span class='msg-avail'>...</span></td><td>Liliana Marie Prikler</td></tr>
<tr><td><span title='2022-10-02 23:18:42 +0200'>2022-10-02</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=782c7455e1ea3785e8c7e9b300d55c9ebb441a35'>gnu: ffmpeg@4: Build against SDL2 2.0 so 'ffplay' gets built.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-09-13 00:20:33 -0400'>2022-09-13</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=f24bf279d41fab25ceb3fcc88bcd5da6fb1404ae'>gnu: sdl2: Update to 2.24.0, use gexps and remove input labels.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2022-09-01 19:08:12 -0400'>2022-09-01</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=bd638a8ad6140db87851a86faae89e736e148f3d'>gnu: Delete mono and dependent packages.</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2022-09-01 16:05:12 +0300'>2022-09-01</span></td><td><a href='/guix/commit/gnu/packages/sdl.scm?id=f04325c92fa8b700619834500e1f3497ac9150b1'>bug#22020: [PATCH] gnu: guile-sdl: Update to 0.6.1.</a><span class='msg-avail'>...</span></td><td>Thiago Jung Bauermann</td></tr>