fix Wunused-but-set-variable warning/error

[lhuang04]

Summary: When the MBEDTLS_DEBUG_C is undefined, we saw the following compiler warnining/error.

/upstream/library/ssl_tls13_keys.c: In function
‘mbedtls_ssl_tls1_3_generate_resumption_master_secret’:
/upstream/library/ssl_tls13_keys.c:890:12: error: variable
‘md_size’ set but not used [-Werror=unused-but-set-variable]
     size_t md_size;

/upstream/library/ssl_tls13_server.c: In function
‘ssl_parse_key_shares_ext’:
/upstream/library/ssl_tls13_server.c:366:39: error:
variable ‘their_curve_info’ set but not used
[-Werror=unused-but-set-variable]
         mbedtls_ecp_curve_info const *their_curve_info;

Test Plan:

rm -rf build/; mkdir build; cd build; CFLAGS="-std=c99 -g
-Wunused-but-set-variable -Wno-unused-parameter" cmake ..; make -j

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Description

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Status

READY/IN DEVELOPMENT/HOLD

Requires Backporting

When there is a bug fix, it should be backported to all maintained and supported branches. Changes do not have to be backported if:

• This PR is a new feature\enhancement
• This PR contains changes in the API. If this is true, and there is a need for the fix to be backported, the fix should be handled differently in the legacy branch

Yes | NO
Which branch?

Migrations

If there is any API change, what's the incentive and logic for it.

YES | NO

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Todos

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

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