ofproto-dpif: Fix use-after-free deleting a bridge with active traffic.

When a bridge that has active traffic is deleted, the bridge's ofproto can
be referenced by in-flight "flow_miss"es.  This commit fixes the problem
by destroying "flow_miss"es that reference the ofproto that is being
deleted.

I found the problem by adding and removing a bridge that had active traffic
(via hping3) while running under valgrind.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Ethan Jackson <ethan@nicira.com>