use mbedtls_ssl_session_init() to init session variable

[BensonLiou]

Description

Use mbedtls_ssl_session_init() to init variable just like session-family APIs described

PR checklist

Please tick as appropriate and edit the reasons (e.g.: "backport: not needed because this is a new feature")

• [ ] changelog provided, or not required
• [ ] backport done, or not required
• [ ] tests provided, or not required

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