Fix xmlParserEntityCheck

A previous commit removed the check for XML_ERR_ENTITY_LOOP which is
required to abort early in case of excessive entity recursion.
diff --git a/parser.c b/parser.c
index 5e800e1..ca9fde2 100644
--- a/parser.c
+++ b/parser.c
@@ -149,7 +149,7 @@
 	rep = xmlStringDecodeEntities(ctxt, ent->content,
 				  XML_SUBSTITUTE_REF, 0, 0, 0);
         --ctxt->depth;
-	if (rep == NULL) {
+	if ((rep == NULL) || (ctxt->errNo == XML_ERR_ENTITY_LOOP)) {
 	    ent->content[0] = 0;
 	}