When master reboot from RDB, if rsi in RDB is valid we should not free replication backlog, even if master_repl_offset or repl-offset is 0.
Since if master doesn't send any data to replicas master_repl_offset is 0, it's a valid number.
A clear example:
1. start a master and apply some write commands, the master's master_repl_offset is 0 since it has no replicas.
2. stop write commands on master, and start another instance and replicaof the master, trigger an FULLRESYNC
3. the master's master_repl_offset is still 0 (set a large number for repl-ping-replica-period), do BGSAVE and restart the master
4. master load master_repl_offset from RDB's rsi and it's still 0, and we should make sure replica can partially resync with master.
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