of13: support OFPAactionExperimenter with data

The length check assert was added by FUJITA since the spec says that
lenght of data should be a multiple of 8.

Signed-off-by: Rob Udechukwu <rnudechu@ncsu.edu>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>