Clang warnings when building with openssl and scrypt

src/test/test_crypto_slow.c:161:23: error: implicit conversion loses integer
      precision: 'uint64_t' (aka 'unsigned long') to 'uint32_t'
      (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
                   N, r, p, buf1, dk_len);
                      ^
src/test/test_crypto_slow.c:161:26: error: implicit conversion loses integer
      precision: 'uint64_t' (aka 'unsigned long') to 'uint32_t'
      (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
                   N, r, p, buf1, dk_len);
                         ^
src/test/test_crypto_slow.c:181:23: error: implicit conversion loses integer
      precision: 'uint64_t' (aka 'unsigned long') to 'uint32_t'
      (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
                   N, r, p, buf1, dk_len);
                      ^
src/test/test_crypto_slow.c:181:26: error: implicit conversion loses integer
      precision: 'uint64_t' (aka 'unsigned long') to 'uint32_t'
      (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
                   N, r, p, buf1, dk_len);
                         ^
src/test/test_crypto_slow.c:204:23: error: implicit conversion loses integer
      precision: 'uint64_t' (aka 'unsigned long') to 'uint32_t'
      (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
                   N, r, p, buf1, dk_len);
                      ^
src/test/test_crypto_slow.c:204:26: error: implicit conversion loses integer
      precision: 'uint64_t' (aka 'unsigned long') to 'uint32_t'
      (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
                   N, r, p, buf1, dk_len);
                         ^
src/test/test_crypto_slow.c:228:23: error: implicit conversion loses integer
      precision: 'uint64_t' (aka 'unsigned long') to 'uint32_t'
      (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
                   N, r, p, buf1, dk_len);
                      ^
src/test/test_crypto_slow.c:228:26: error: implicit conversion loses integer
      precision: 'uint64_t' (aka 'unsigned long') to 'uint32_t'
      (aka 'unsigned int') [-Werror,-Wshorten-64-to-32]
                   N, r, p, buf1, dk_len);