ofproto: Fix using uninitialized delete_reason.

ofproto: Fix using uninitialized delete_reason.

replace_rule_finish() makes decision using uninitialized
for intrenal flow fm->delete_reason.
Reported by valgrind for test cases 886, 942 and 943.

 Conditional jump or move depends on uninitialised value(s)
    at rule_insert (ofproto-dpif.c:4134)
    by replace_rule_finish (ofproto.c:4831)
    by add_flow_finish (ofproto.c:4661)
    by modify_flows_finish (ofproto.c:4994)
    by ofproto_flow_mod_finish (ofproto.c:6821)
    by handle_flow_mod__ (ofproto.c:5323)
    by ofproto_dpif_add_internal_flow (ofproto-dpif.c:5680)
    by add_internal_miss_flow (ofproto-dpif.c:1385)
    by add_internal_flows (ofproto-dpif.c:1412)
    by construct (ofproto-dpif.c:1367)
    by ofproto_create (ofproto.c:577)
    by bridge_reconfigure (bridge.c:633)
    by bridge_run (bridge.c:2975)
    by main (ovs-vswitchd.c:120)
  Uninitialised value was created by a stack allocation
    at ofproto_dpif_add_internal_flow (ofproto-dpif.c:5658)

Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>