libfetch: Check for failure to create SSL context

• Drop the ssl_meth member, there is no reason to hang on to it.

• Replace deprecated SSLv23_client_method() with TLS_client_method().

• Check the return value from SSL_CTX_new().

MFC after: 1 week
PR: 292903
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D55098