[libxml2] Fix sanity check in htmlParseNameComplex

[Nick Wellnhofer <nwellnhof src gnome org>]

commit f39e3be0dd9b3aa3ec47ff339b4934c16f6e5156
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Sun Jun 11 12:35:59 2017 +0200

    Fix sanity check in htmlParseNameComplex
    
    - (cur - len) can overflow.
    - Throw an internal error.
    
    Fixes bug 780077.

 HTMLparser.c |    8 ++++++--
 1 files changed, 6 insertions(+), 2 deletions(-)
---
diff --git a/HTMLparser.c b/HTMLparser.c
index d1395fa..3198afa 100644
--- a/HTMLparser.c
+++ b/HTMLparser.c
@@ -2528,8 +2528,12 @@ htmlParseNameComplex(xmlParserCtxtPtr ctxt) {
        }
     }
 
-    if (ctxt->input->base > ctxt->input->cur - len)
-       return(NULL);
+    if (ctxt->input->cur - ctxt->input->base < len) {
+        /* Sanity check */
+       htmlParseErr(ctxt, XML_ERR_INTERNAL_ERROR,
+                     "unexpected change of input buffer", NULL, NULL);
+        return (NULL);
+    }
 
     return(xmlDictLookup(ctxt->dict, ctxt->input->cur - len, len));
 }