IncHashSlotCount fix

[zhhlovely]

What is the purpose of the change

修复索引槽位已使用统计问题，IncHashSlotCount的增加应该是判断这个槽位为空的时候才应该进行+1操作，如果查询发现不为空，即代表之前这个槽之前已经被占领了，就不应该进行IncHashSlotCount增加操作。

Brief changelog

IndexFile putKey(final String key, final long phyOffset, final long storeTimestamp) 进行了修改
XX

Verifying this change

XXXX

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[xujianhai666]

@zhhlovely ci err, fix firstly. Besides, I thought the hashSlot is the number of slot