Don't skip leading zeroes in PSK keys.

Don't use BN_hex2bn() for PSK key conversion as the conversion to
BN and back removes leading zeroes, use OPENSSL_hexstr2buf() instead.

RT#4554

Reviewed-by: Matt Caswell <matt@openssl.org>
(cherry picked from commit 6ec6d520)

Conflicts:
	apps/s_client.c
	apps/s_server.c