scan_asn1derlength decodes an unsigned integer in ASN.1 DER length encoding
from a memory area holding binary data. It writes the decode value in
dest and returns the number of bytes it read from src.
scan_asn1derlength never reads more than len bytes from src. If the
sequence is longer than that, or the memory area contains an invalid
sequence, scan_asn1derlength returns 0 and does not touch dest.
The length of the longest ASN.1 DER length sequence is 128 bytes. In
practice the largest sequence is sizeof(*dest)+1.