This function will convert the given data to printable data, using
the base64 encoding, as used in the libsrp. This is the encoding
used in SRP password files. If the provided buffer is not long
enough GNUTLS_E_SHORT_MEMORY_BUFFER is returned.
Warning! This base64 encoding is not the "standard" encoding, so
do not use it for non-SRP purposes.
GNUTLS_E_SHORT_MEMORY_BUFFER if the buffer given is not
long enough, or 0 on success.