Fix usage of BIO_get_mem_data

[samuel40791765]

According to the function signature in OpenSSL's documentation, the BIO_get_mem_data function should take a char ** argument instead of a const char **. const char** and char ** aren't directly interchangeable, and using const char** is considered a misusage.

Reference: https://www.openssl.org/docs/man1.1.1/man3/BIO_get_mem_data.html

I am contributing on behalf of my employer Amazon Web Services.

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[samuel40791765]

I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

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