Don't call memcpy if len is zero.

Prevent undefined behavior in CRYPTO_cbc128_encrypt: calling this function
with the 'len' parameter being 0 would result in a memcpy where the source
and destination parameters are the same, which is undefined behavior.
Do same for AES_ige_encrypt.

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2671)