Fix OpenSSL build with GCC v15.1.1 [-Wformat-truncation=] (#2077)

Fix OpenSSL build with GCC v15.1.1 [-Wformat-truncation=] (#2077)

On arm64 Fedora 42:

    src/ssl/crtd_message.cc:132:39: error: '%zd' directive output may be
        truncated writing between 1 and 19 bytes into a region
        of size 10 [-Werror=format-truncation=]
        snprintf(buffer, sizeof(buffer), "%zd", body.length());