Update server.lastbgsave_status when fork() fails.

src/rdb.c

@@ -746,6 +746,7 @@ int rdbSaveBackground(char *filename) {
         /* Parent */
         server.stat_fork_time = ustime()-start;
         if (childpid == -1) {
+            server.lastbgsave_status = REDIS_ERR;
             redisLog(REDIS_WARNING,"Can't save in background: fork: %s",
                 strerror(errno));
             return REDIS_ERR;