fmt_asn1derlength encodes an unsigned integer using the ASN.1 DER
for encoding tag lengths. This can take from 1 byte (0-0x7f) up to
If dest equals FMT_LEN (i.e. is NULL), fmt_asn1derlength returns the
number of bytes it would have written.
For convenience, fmt.h defines the integer FMT_ASN1LENGTH to be big
enough to contain every possible fmt_asn1derlength output.