Fix a wrong test

https://bugzilla.gnome.org/show_bug.cgi?id=704530

Arg is an array on the stack and can't be NULL
diff --git a/debugXML.c b/debugXML.c
index c8efe6a..3c13783 100644
--- a/debugXML.c
+++ b/debugXML.c
@@ -2946,7 +2946,7 @@
         } else if (!strcmp(command, "save")) {
             xmlShellSave(ctxt, arg, NULL, NULL);
         } else if (!strcmp(command, "write")) {
-	    if ((arg == NULL) || (arg[0] == 0))
+	    if (arg[0] == 0)
 		xmlGenericError(xmlGenericErrorContext,
                         "Write command requires a filename argument\n");
 	    else