openvswitch: Avoid memory corruption in queue_userspace_packet()

In queue_userspace_packet(), the ovs_nla_put_flow return value is
not checked. This is fine as long as key_attr_size() returns the
correct value. In case it does not, the current code may corrupt buffer
memory. Add a run time assertion catch this case to avoid silent
failure.

Reported-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Andy Zhou <azhou@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>