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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<tr><td><span title='2020-07-12 23:08:15 +0200'>2020-07-12</span></td><td><a href='/guix/commit/gnu/packages/avr.scm?id=b0e7b6992f3f845e83cfbca4d700b51dba50b4d5'>gnu: Remove ".git" from "https://github/…/….git".</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-05-13 21:07:40 +0200'>2020-05-13</span></td><td><a href='/guix/commit/gnu/packages/avr.scm?id=fd96ae4a709c5509f2d78af3968d64fe805ad1ef'>gnu: avr-gcc: Fix conflicting GCC headers.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2020-03-01 12:52:43 +0100'>2020-03-01</span></td><td><a href='/guix/commit/gnu/packages/avr.scm?id=2f3515f7369d91d8922620044f9348cc727b6c10'>gnu: avr-gcc: Fix compiler lookup paths.</a><span class='msg-avail'>...</span></td><td>Danny Milosavljevic</td></tr>