Add a check to prevent len from going negative in xmlParseAttValueComplex.

third_party/libxml/README.chromium

 Name: libxml
 URL: http://xmlsoft.org
 Version: 2.7.7
 License: MIT
 License File: src/Copyright
 Security Critical: yes
 
 Description:
 The src/ directory contains a partial snapshot of the libxml library
 with the patches in the patches/ directories applied.
@@ skipping 14 matching lines @@
 - Import UTF-8 fix from upstream: http://git.gnome.org/browse/libxml2/commit/?id=0795348aeb86648723bc391e4d02e20631c10bca 
 - Import XPath fix http://git.gnome.org/browse/libxml2/commit/xpath.c?id=2ddecc23862bab1a9a9e51e097aefc92ec305e28
 - Merge clang warning fix http://git.gnome.org/browse/libxml2/commit/?id=aae48e64dfbf2b46b157a4c1857e30645116388f
 - Add a fix for proper escaping of xpointer expressions, commit upstream is pending.
 - Add helper classes in chromium/libxml_utils.cc and chromium/include/libxml/libxml_utils.h.
 - Add a tweak to limit problems caused by excessive strings and buffers.
 - Change the xmlNs struct a little bit, so it looks like it has no children
 if treated as a generic xmlNode object.
 - Fix pretty harmless use-after-free in generate-id function.
 - Merge a clang warning fix http://git.gnome.org/browse/libxml2/commit/?id=713434d2309da469d64b35e163ea6556dadccada
+- Import attribute normalization fix http://git.gnome.org/browse/libxml2/commit/?id=6a36fbe3b3e001a8a840b5c1fdd81cefc9947f0d
 
 To import a new snapshot of libxml:
 
 - Visit http://xmlsoft.org/downloads.html and download the latest source
   distribution.
 - Copy the files into this directory, omitting files which have been omitted
   here.  E.g.:  for i in $(find . -type f); do cp ../libxml-newver/$i $i; done
   This should clobber all local changes to this directory.
 - Apply the patches in patches/ and fix any problems.
   UPDATE THOSE PATCHES OR EVAN WILL HUNT YOU DOWN.
 - On a Linux system,
     $ cd linux
     $ ../configure --without-iconv --without-ftp --without-http
   to generate config.h and include/libxml/xmlversion.h for Linux.
 - On a Mac,
     $ cd mac
     $ ../configure --without-iconv --without-ftp --without-http
   to generate config.h and include/libxml/xmlversion.h for Macs.
 - On Windows, run build/generate-win32-headers.bat to re-generate config.h and
   include/libxml/xmlversion.h for Windows builds.
 - Update this README to reflect the new version number.