fix const mismatch for pointer to data (#974)

See [SSL_get_tlsext_status_ocsp_resp](https://www.openssl.org/docs/man3.0/man3/SSL_get_tlsext_status_ocsp_resp.html)

The `resp` argument is not `const`, and when `mongoc-openssl.c` is built on Windows this causes a [C4090](https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4090) error.

We can work around this by simply passing a pointer to a pointer that is not const, and then assigning its value to the const pointer.

Author: TheDorkKnight

src/libmongoc/src/mongoc/mongoc-openssl.c

@@ -775,6 +775,7 @@ _mongoc_ocsp_tlsext_status (SSL *ssl, mongoc_openssl_ocsp_opt_t *opts)
    X509 *peer = NULL, *issuer = NULL;
    STACK_OF (X509) *cert_chain = NULL;
    const unsigned char *resp_data = NULL;
+   unsigned char *mutable_resp_data = NULL;
    int cert_status, reason, len, status;
    OCSP_CERTID *id = NULL;
    ASN1_GENERALIZEDTIME *produced_at = NULL, *this_update = NULL,
@@ -817,7 +818,8 @@ _mongoc_ocsp_tlsext_status (SSL *ssl, mongoc_openssl_ocsp_opt_t *opts)
    }
 
    /* Get the stapled OCSP response returned by the server */
-   len = SSL_get_tlsext_status_ocsp_resp (ssl, &resp_data);
+   len = SSL_get_tlsext_status_ocsp_resp (ssl, &mutable_resp_data);
+   resp_data = mutable_resp_data;
    stapled_response = !!resp_data;
    if (stapled_response) {
       /* obtain an OCSP_RESPONSE object from the OCSP response */