Update libxml to 2.7.7.

third_party/libxml/catalog.c

Index: third_party/libxml/catalog.c
diff --git a/third_party/libxml/catalog.c b/third_party/libxml/catalog.c
index ee3f8f2bbbab3f2c1099bafe6dfcf0641820774c..af84b7c4bc63c69e6e1eb1ab802d0acdb7244a3b 100644
--- a/third_party/libxml/catalog.c
+++ b/third_party/libxml/catalog.c
@@ -2616,6 +2616,8 @@ xmlCatalogSGMLResolve(xmlCatalogPtr catal, const xmlChar *pubID,
 	return(ret);
     if (sysID != NULL)
 	ret = xmlCatalogGetSGMLSystem(catal->sgml, sysID);
+    if (ret != NULL)
+	return(ret);
     return(NULL);
 }
 
@@ -2912,7 +2914,7 @@ xmlACatalogResolveURI(xmlCatalogPtr catal, const xmlChar *URI) {
 
 	sgml = xmlCatalogSGMLResolve(catal, NULL, URI);
 	if (sgml != NULL)
-            sgml = xmlStrdup(sgml);
+            ret = xmlStrdup(sgml);
     }
     return(ret);
 }