SAX2: Reenable 'directory' as base URI fallback

Apparently, some users overwrite this member manually to set a base URI
for memory streams.

Fixes #753.

Author: nwellnhof

SAX2.c

@@ -412,6 +412,13 @@ xmlSAX2ResolveEntity(void *ctx, const xmlChar *publicId, const xmlChar *systemId
     if (ctxt->input != NULL)
 	base = BAD_CAST ctxt->input->filename;
 
+    /*
+     * We don't really need the 'directory' struct member, but some
+     * users set it manually to a base URI for memory streams.
+     */
+    if (base == NULL)
+        base = BAD_CAST ctxt->directory;
+
     if ((xmlStrlen(systemId) > XML_MAX_URI_LENGTH) ||
         (xmlStrlen(base) > XML_MAX_URI_LENGTH)) {
         xmlFatalErr(ctxt, XML_ERR_RESOURCE_LIMIT, "URI too long");
@@ -574,6 +581,13 @@ xmlSAX2EntityDecl(void *ctx, const xmlChar *name, int type,
             }
         }
 
+        /*
+         * We don't really need the 'directory' struct member, but some
+         * users set it manually to a base URI for memory streams.
+         */
+        if (base == NULL)
+            base = ctxt->directory;
+
         res = xmlBuildURISafe(systemId, (const xmlChar *) base, &URI);
 
         if (URI == NULL) {