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;
ubstitutes?id=9022861dc028e99fab930721fe991a682c497bbb'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/etc/substitutes'>
<input type='hidden' name='id' value='9022861dc028e99fab930721fe991a682c497bbb'/><select name='qt'>
<option value='grep'>log msg</option>
<option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=9022861dc028e99fab930721fe991a682c497bbb'>root</a>/<a href='/guix/log/etc?id=9022861dc028e99fab930721fe991a682c497bbb'>etc</a>/<a href='/guix/log/etc/substitutes?id=9022861dc028e99fab930721fe991a682c497bbb'>substitutes</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/etc/substitutes?id=9022861dc028e99fab930721fe991a682c497bbb&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-07-10 00:11:00 +0200'>2020-07-10</span></td><td><a href='/guix/commit/etc/substitutes?id=d283bb960f927dd5f7bb8b96bc697221e4e8ad39'>maint: Remove traces of "berlin.guixsd.org".</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-06-29 14:27:22 +0200'>2019-06-29</span></td><td><a href='/guix/commit/etc/substitutes?id=3a8bfebed94ba295432da8304b7ffb4a611ef761'>etc: Remove hydra.gnu.org.pub key.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2019-05-01 11:05:47 +0200'>2019-05-01</span></td><td><a href='/guix/commit/etc/substitutes?id=757e633d57a7b994759d69e41dd759a39f1a34ae'>build: Change default substitute server to "ci.guix.gnu.org".</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>