builtin-reflog.c: fix typo that accesses an unset variable

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-reflog.c

@@ -394,7 +394,7 @@ static int cmd_reflog_delete(int argc, const char **argv, const char *prefix)
 		int recno;
 
 		if (!spec) {
-			status |= error("Not a reflog: %s", ref);
+			status |= error("Not a reflog: %s", argv[i]);
 			continue;
 		}