applied patch from Malcolm Tredinnick fixing bug #152426 Daniel

* parser.c: applied patch from Malcolm Tredinnick fixing bug #152426
Daniel
diff --git a/parser.c b/parser.c
index e1523c0..2b8f5ed 100644
--- a/parser.c
+++ b/parser.c
@@ -10219,7 +10219,7 @@
      * Canonicalise the system ID
      */
     systemIdCanonic = xmlCanonicPath(SystemID);
-    if (systemIdCanonic == NULL) {
+    if ((SystemID != NULL) && (systemIdCanonic == NULL)) {
 	xmlFreeParserCtxt(ctxt);
 	return(NULL);
     }