Don't remove leading zero byte.

* src/cms.c (ksba_cms_set_sig_val): Don't remove leading zero byte.
(ksba_cms_set_enc_val): Likewise.
* src/certreq.c (ksba_certreq_set_sig_val): Likewise for RSA.

For RSA, here is in PKCS1 format with fixed-size. There should not be
an extra leading zero byte. (We used to have an extra leading zero
byte with smartcard.)

• GnuPG-bug-id: T4104
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>