datapath: Fix Flow dump operation.

Following patch adds null check while inserting new netlink attribute.
This was introduced by commit 9b405f1aa8d175d (datapath: More
flexible kernel/userspace tunneling attribute.)

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>
Bug #14767