ofproto-dpif-upcall: Don't use stack garbage

Catched by "learning action - self-modifying flow with hard_timeout"
test case.

The bug introduced by commit b256dc52.
("ofproto-dpif-xlate: Cache xlate_actions() effects.")

Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>