RSA: avoid dereferencing possibly-NULL parameter in initializers
RSA: avoid dereferencing possibly-NULL parameter in initializers

Fix CID 1472835: the explicit NULL check for prsactx is useless when
we have already dereferenced it in the initializers.

Move the actual initialization to the function body to get the
logic sequenced properly.

Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
Reviewed-by: Shane Lontis <452ce16516ceed26291fe7de3f8b53540d83864e@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/14160)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl