providers/implementations/keymgmt/rsa_kmgmt.c: refactor gen_init

There is risk to pass the gctx with NULL value to rsa_gen_set_params
which dereference gctx directly.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17429)