Stéphane Bidoul found an off by one addressing error on the error

* relaxng.c: Stéphane Bidoul found an off by one addressing
  error on the error handling.
Daniel
diff --git a/relaxng.c b/relaxng.c
index 55e0852..99ff9e9 100644
--- a/relaxng.c
+++ b/relaxng.c
@@ -1991,7 +1991,7 @@
     if (msg[0] == 0) {
 	snprintf(msg, 1000, "Unknown error code %d", err);
     }
-    msg[1000] = 0;
+    msg[1000 - 1] = 0;
     return(xmlStrdup((xmlChar *) msg));
 }