Make RSA_generate_multi_prime_key() not segfault if e is NULL.

Make RSA_generate_multi_prime_key() not segfault if e is NULL.

This is not a big problem for higher level keygen, as these set e
beforehand to a default value. But the logic at the lower level is
incorrect since it was doing a NULL check in one place but then
segfaulting during a later BN_copy().

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Todd Short <todd.short@me.com>
(Merged from https://github.com/openssl/openssl/pull/20025)