ofproto: Correct encoding and decoding of group desc properties.

ofproto: Correct encoding and decoding of group desc properties.

* encode: if properties are present include their length in
          value of the length field of the group desc
* decode: use the value of the length field to calculate the length of
          properties rather than assuming that the rest of the message
          is properties. This assumption is not correct as a message
          may contain multiple group descs.

Fixes: 18ac06d3546e ("ofp-util: Encoding and decoding of (draft) OpenFlow 1.5 group messages.")
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>