set aof rewrite status err, when fork fail (#5606)

when aof rewrite is failed by fork(), It'll be indicated by aof_last_bgrewrite_status
INFO field, same as when the fork child fails later on.

src/aof.c

@@ -1755,6 +1755,7 @@ int rewriteAppendOnlyFileBackground(void) {
     } else {
         /* Parent */
         if (childpid == -1) {
+            server.aof_lastbgrewrite_status = C_ERR;
             serverLog(LL_WARNING,
                 "Can't rewrite append only file in background: fork: %s",
                 strerror(errno));