gnutls: prevent buffer overflow in get_cert_name
The test suite for ocserv calls openconnect with a certificate that has a name that is 84 bytes in length. The buffer passed to get_cert_name is currently 80 bytes.
gnutls_x509_crt_get_dn_by_oid function will update the buffer size
parameter if the buffer is too small.
RETURNS GNUTLS_E_SHORT_MEMORY_BUFFER if the provided buffer is not long enough, and in that case the buf_size will be updated with the required size. GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE if there are no data in the current index. On success 0 is returned.
Use a temporary variable to avoid clobbering the
namelen variable that is