[libxml2] Fix memory leak in xmlParseElementMixedContentDecl

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 45da175c1431d69e74e05a115f0b14cc8c97d886
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Fri Dec 18 12:14:52 2020 +0100

    Fix memory leak in xmlParseElementMixedContentDecl
    
    Free parsed content if malloc fails to avoid a memory leak.
    
    Found with libFuzzer.

 parser.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)
---
diff --git a/parser.c b/parser.c
index 85494df4..43b88358 100644
--- a/parser.c
+++ b/parser.c
@@ -6082,14 +6082,20 @@ xmlParseElementMixedContentDecl(xmlParserCtxtPtr ctxt, int inputchk) {
            NEXT;
            if (elem == NULL) {
                ret = xmlNewDocElementContent(ctxt->myDoc, NULL, XML_ELEMENT_CONTENT_OR);
-               if (ret == NULL) return(NULL);
+               if (ret == NULL) {
+                   xmlFreeDocElementContent(ctxt->myDoc, cur);
+                    return(NULL);
+                }
                ret->c1 = cur;
                if (cur != NULL)
                    cur->parent = ret;
                cur = ret;
            } else {
                n = xmlNewDocElementContent(ctxt->myDoc, NULL, XML_ELEMENT_CONTENT_OR);
-               if (n == NULL) return(NULL);
+               if (n == NULL) {
+                   xmlFreeDocElementContent(ctxt->myDoc, ret);
+                    return(NULL);
+                }
                n->c1 = xmlNewDocElementContent(ctxt->myDoc, elem, XML_ELEMENT_CONTENT_ELEMENT);
                if (n->c1 != NULL)
                    n->c1->parent = n;