index-pack: report error using the correct variable

We feed a string pointer that is potentially NULL to die() when
showing the message.  Don't.

Noticed-by: Nguyễn Thái Ngọc Duy  <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/index-pack.c

@@ -1291,15 +1291,15 @@ static void final(const char *final_pack_name, const char *curr_pack_name,
 		if (keep_fd < 0) {
 			if (errno != EEXIST)
 				die_errno(_("cannot write keep file '%s'"),
-					  keep_name);
+					  keep_name ? keep_name : name);
 		} else {
 			if (keep_msg_len > 0) {
 				write_or_die(keep_fd, keep_msg, keep_msg_len);
 				write_or_die(keep_fd, "\n", 1);
 			}
 			if (close(keep_fd) != 0)
 				die_errno(_("cannot close written keep file '%s'"),
-				    keep_name);
+					  keep_name ? keep_name : name);
 			report = "keep";
 		}
 	}