initialize auth_key_length_ in MySqlHostExchange Constructor

CID 1476326 (#1 of 1): Uninitialized scalar field (UNINIT_CTOR)
2. uninit_member: Non-static class member auth_key_length_ is not initialized in this constructor nor in any functions that it calls.

Edited May 11, 2021 by Razvan Becheriu

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