Length fields are not being checked

[ajoaoff]

Original issue opened by @erickvermot at kytos#314.

length fields are not beeing checked.
(pg 50) - "OpenFlow is a simple binary protocol, therefore invalid OpenFlow messages
generated by OpenFlow implementations will in general result in the message
having the wrong length or some fields having invalid values (see 7.1.3). It
is recommended to monitor those conditions to detect non-compliant
implementations."

• when length is passed as a parameter, as in flow_match, maybe length
  should be computed locally, or checked for validity before message is sent.