apps/speed.c: add missing checks for RAND_bytes()

Function RAND_bytes() may return 0 or -1 on error, simply
goto end label when it fails.

Fixes #6567

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/6582)