apps/speed.c: Fix crash when config loading fails

Move rsa_key initialization in front of load_config().
If loading the config fails, rsa_key isn't initialized and may
cause invalid free() in the end: cleanup.
Remove superfluous memset.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>