Don't use chunk_skip() in asn1_length().

Don't use chunk_skip() in asn1_length().

chunk_skip() returns chunk_empty if the length of the chunk is equal to
the number of bytes to skip, this is problematic as asn1_length() modifies
the original chunk.  asn1_parser_t for instance uses the modified chunk to
later calculate the length of the resulting ASN.1 object which produces
incorrect results if it is based on chunk_empty.