when the data block ends with SPACEs or NULs.

The problem is, you can't see if the data ends
with SPACE or NUL or a combination of both.

This can happen for instance with
openssl rsautl -decrypt -hexdump
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5328)