Bugfix: duplicate match fields incorrect length.
Bugfix: duplicate match fields incorrect length.

Previous fix prevented the segfault, but duplicate matches were still
causing incorrect OpenFlow flowmods on the wire, causing error reply
with type=OFPET_BAD_MATCH and code=OFPBMC_BAD_LEN. This changeset
ignores calls to duplicate fields altogether, and now works correctly on
the wire.

ARCCN/libfluid_msg,ARCCN/libfluid_msg,OpenNetworkingFoundation/libfluid_msg,OpenNetworkingFoundation/libfluid_msg,ARCCN/libfluid_msg,OpenNetworkingFoundation/libfluid_msg