crypto: Improve PER OID calculations

crypto: Improve PER OID calculations

"(oid[0] << 4) & (oid[1] & 0x0F)" statement is always 0. It is not
problem currently because the only OID which is written by this
function should have 0 there. The function to read/write are pretty
limited anyway and can't work properly with all kind of OIDs. Maybe
it would be better to hardcode the OID there without decoding
and encoding. But those functions are already there so let's improve
them a bit according the spec and warn about limited set of
supported OIDs.

See:
https://msdn.microsoft.com/en-us/library/windows/desktop/bb540809