Fix key wrapping mode with padding to conform to RFC 5649.

According to RFC 5649 section 4.1 step 1) we should not add padding
if plaintext length is multiply of 8 ockets.

This matches pseudo-code in http://dx.doi.org/10.6028/NIST.SP.800-38F


on page 15, section 6.3 KWP, algorithm 5 KWP-AE, step 2.

PR#3675

Reviewed-by: Stephen Henson <steve@openssl.org>
Reviewed-by: Andy Polyakov <appro@openssl.org>