a pointer to a structure to hold the fingerprint (may be null)
size_t * sizeof_buf
initially holds the size of buf
DESCRIPTION
This function will calculate and copy the certificate's fingerprint
in the provided buffer.
If the buffer is null then only the size will be filled.
RETURNS
GNUTLS_E_SHORT_MEMORY_BUFFER if the provided buffer is
not long enough, and in that case the *sizeof_buf will be updated
with the required size. On success 0 is returned.