fix inconsistent flen check in rsa_pk1 and rsa_oaep

Fixes #7117

Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7880)

(cherry picked from commit 4fea7005)