gpg: Don't try decryption by session key when NULL.
* g10/mainproc.c (proc_encrypted): Only call get_session_key when PKENC_LIST is not NULL. Return GPG_ERR_BAD_KEY, instead of GPG_ERR_NO_SECKEY, when it's encrypted only by symmetric key.
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>