[libxml2] Fix infinite loop in xmlStringLenDecodeEntities

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 0e1a49c8907645d2e155f0d89d4d9895ac5112b5
Author: Zhipeng Xie <xiezhipeng1 huawei com>
Date:   Thu Dec 12 17:30:55 2019 +0800

    Fix infinite loop in xmlStringLenDecodeEntities
    
    When ctxt->instate == XML_PARSER_EOF,xmlParseStringEntityRef
    return NULL which cause a infinite loop in xmlStringLenDecodeEntities
    
    Found with libFuzzer.
    
    Signed-off-by: Zhipeng Xie <xiezhipeng1 huawei com>

 parser.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/parser.c b/parser.c
index d1c31963..a34bb6cd 100644
--- a/parser.c
+++ b/parser.c
@@ -2646,7 +2646,8 @@ xmlStringLenDecodeEntities(xmlParserCtxtPtr ctxt, const xmlChar *str, int len,
     else
         c = 0;
     while ((c != 0) && (c != end) && /* non input consuming loop */
-          (c != end2) && (c != end3)) {
+           (c != end2) && (c != end3) &&
+           (ctxt->instate != XML_PARSER_EOF)) {
 
        if (c == 0) break;
         if ((c == '&') && (str[1] == '#')) {