The return value of RSA_*_{en,de}crypt() is signed

The return value of RSA_*_{en,de}crypt() is signed

The functions RSA_(public|private)_(en|de)crypt() return a signed
result, in particular `-1` may be returned on error, so the caller
MUST treat the value as signed.

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/29323)