bridge: Fix null dereference on ct_timeout_policy record

bridge: Fix null dereference on ct_timeout_policy record

Accoridng to vswitch.ovsschema, each CT_Zone record may have
zero or one associcated CT_Timeout_policy.  Thus, this patch
checks if ovsrec_ct_timeout_policy exist before accesses the
record.

VMWare-BZ: 2585825
Fixes: 45339539f69d ("ovs-vsctl: Add conntrack zone commands.")
Fixes: 993cae678bca ("ofproto-dpif: Consume CT_Zone, and CT_Timeout_Policy tables")
Reported-by: Yang Song <yangsong@vmware.com>
Signed-off-by: Yi-Hung Wei <yihung.wei@gmail.com>
Signed-off-by: William Tu <u9012063@gmail.com>