crypto/evp/e_aes.c: add size_t casts to increase readability

Signed-off-by: Patrick Steuer <patrick.steuer@de.ibm.com>

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5250)