Fix memory leak in mbedtls_ssl_handshake_free

[lhuang04]

Summary: We try to mbedtls_free object inside handshake after its address is zeroed out by mbedtls_platform_zeroize

Test Plan: Here is from our report.

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==3365627==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 256 byte(s) in 1 object(s) allocated from:
    #0 0x1ab37c7 in calloc (/data/users/lhuang04/fbsource/fbcode/buck-out/dbg/cells/fbsource/gen/aab7ed39/xplat/mobilenetwork/test/test+0x1ab37c7)
    #1 0x10f549d in ssl_server_hello_postprocess xplat/mobilenetwork/third-party/mbedtls/library/ssl_tls13_client.c:3369
    #2 0x10e7e2e in ssl_server_hello_process xplat/mobilenetwork/third-party/mbedtls/library/ssl_tls13_client.c:2864
    #3 0x10e6b6c in mbedtls_ssl_handshake_client_step_tls1_3 xplat/mobilenetwork/third-party/mbedtls/library/ssl_tls13_client.c:4175
    #4 0x10dfd77 in mbedtls_ssl_handshake_step xplat/mobilenetwork/third-party/mbedtls/library/ssl_tls.c:6090

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• [ ] Tests
• [ ] Documentation
• [ ] Changelog updated
• [ ] Backported

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