malloc-fail: Fix memory leak in xmlCreatePushParserCtxt

Found with libFuzzer, see #344.

parser.c

@@ -12577,6 +12577,7 @@ xmlCreatePushParserCtxt(xmlSAXHandlerPtr sax, void *user_data,
 	inputStream->filename = (char *)
 	    xmlCanonicPath((const xmlChar *) filename);
 	if (inputStream->filename == NULL) {
+            xmlFreeInputStream(inputStream);
 	    xmlFreeParserCtxt(ctxt);
 	    xmlFreeParserInputBuffer(buf);
 	    return(NULL);