Don't accept tag number 31 (long form identifier octets) in CBB_add_asn1.

Tag number 31 is a long form tag that requires multiple octets.  It
cannot be handled by adding a single uint8.  Changed CBB_add_asn1()
to return 0 when it is passed in the extension for tag 31.

Change-Id: Ia33936d4f174d1a7176eb11da0b5c7370efb9416