Fail decoding base64 of invalid length.

olm::decode_base64 now returns the length of the raw decoded data on
success. When given input with an invalid base64 length, it fails early
(before decoding any input) and returns -1.

This also makes the C function _olm_decode_base64 an actual binding of
olm::decode_base64 instead of a wrapper with slightly different
behaviour.