Fix bad cast in SSHKDF

[justsmth]

Issues:

N/A

Description of changes:

• Bad cast was causing SSHKDF to break on ppc64be.
  • The [third parameter of EVP_DigestFinal_ex]

    aws-lc/include/openssl/digest.h

    Line 184 in bb1aaba

    OPENSSL_EXPORT int EVP_DigestFinal_ex(EVP_MD_CTX *ctx, uint8_t *md_out,

    is expected to be an unsigned int *, but a size_t* was being passed.

Call-outs:

N/A

Testing:

❯ ppc64-qemu.sh ./crypto/crypto_test --gtest_filter="SSHKDFTest.*"  
Note: Google Test filter = SSHKDFTest.*
[==========] Running 2 tests from 1 test suite.
[----------] Global test environment set-up.
[----------] 2 tests from SSHKDFTest
[ RUN      ] SSHKDFTest.SSHKDF_INPUT_INSANITY
[       OK ] SSHKDFTest.SSHKDF_INPUT_INSANITY (3 ms)
[ RUN      ] SSHKDFTest.KAT
[       OK ] SSHKDFTest.KAT (388 ms)
[----------] 2 tests from SSHKDFTest (393 ms total)

[----------] Global test environment tear-down
[==========] 2 tests from 1 test suite ran. (401 ms total)
[  PASSED  ] 2 tests.

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