Use EVP_PKEY_size() to allocate correct size of signature buffer. (#18)

Do not use fixed buffer size for signature, EVP_SignFinal() requires
buffer for signature at least  EVP_PKEY_size(pkey) bytes in size.

Fixes crash when using 4K RSA signatures (https://github.com/OpenSC/pam_p11/issues/16, https://github.com/OpenSC/pam_p11/issues/15)
PAM_EXTERN int pam_sm_setcred(pam_handle_t * pamh, int flags, int argc,
		const char **argv)
{
	/* Actually, we should return the same value as pam_sm_authenticate(). */
	return PAM_SUCCESS;
}

PAM_EXTERN int pam_sm_setcred(pam_handle_t * pamh, int flags, int argc,
		const char **argv)
{
	/* Actually, we should return the same value as pam_sm_authenticate(). */
	return PAM_SUCCESS;
}
