ofproto-dpif-upcall: Fix logic error in handler/revalidator threads
creation and deletion.

Commit 1f8675481e (ofproto-dpif-upcall: Fix ovs-vswitchd crash.)
directly copied the udpif_set_threads() logic to udpif_stop_threads()
and udpif_start_threads().  In fact, this was erroneous and caused
unittest failures.

This commit fixes the above issue by correcting the checks in
udpif_stop_threads() and udpif_start_threads(), and adding necessary
checks in udpif_set_threads().

Acked-by: Ethan Jackson <ethan@nicira.com>
Signed-off-by: Alex Wang <alexw@nicira.com>