revalidator: Fix access of uninitialized memory.

Commit 64bb477 "dpif: Minimize memory copy for revalidation." introduced
a bug where the corner case of ukey creation (in revalidator threads)
could result in access to uninitialized memory when deleting flows from
the datapath. This could result in OVS aborting or deadlock. Fix it.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>