ofproto-dpif: Fix 'size' argument to fix_sflow_action().

ofproto-dpif: Fix 'size' argument to fix_sflow_action().

The sflow action only uses 8 bytes of the total 16 for user_action_cookie,
but fix_sflow_action() was checking for the presence of all 16, so if the
sflow action wasn't followed by a few other actions then 'cookie' would
end up NULL and the assertion would segfault.

Bug #16659.
Reported-by: Dhaval Badiani <dbadiani@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>