Fix message parsing for length-sensitive types

Certain LDAP messages (such as DeleteRequest) encode their contents as
raw bytes within the top-level sequence object.  As such, they rely
their length being passed to them when LDAPMessage decodes the sequence.

This was being done incorrectly, but would not manifest itself as a
problem unless controls followed the message.  If no controls were
present, then length of the sequence item was bounded by the message
itself and the parse would succeed.

Fix mcavage/node-ldapjs#212