rsa: check that pointer checksum isn't NULL before using it

The pointer checksum were used before checking that it
isn't NULL. We move the code that use it after the check.

Reported-by: Coverity (CID: 185835)
Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>