"Put" watch events with nil value cause SIGSEGV

[dzvancuks]

1. Setup https://github.com/ligato/vpp-agent to watch Redis "config/vpp/v2/interfaces" events. Set up Redis to notify all events CONFIG SET notify-keyspace-events AKE
2. Create many Put watch events by writing some values into Redis /vnf-agent/$mslbl/config/vpp/v2/interfaces/. E.g. 4000 GRE tunnels.
3. Immediately (<1sec) after Redis provisioning do launch Lua script that deletes these keys EVAL "return redis.call('del', unpack(redis.call('keys', ARGV[1])))" 0 /vnf-agent/*/config/vpp/v2/interfaces/*.

According to redis-cli watch CN-Infra has queue of keys to notify VPP-Agent with Put events. On each event it separately reads value for given key. But since Lua script blocks Redis and wipes all keys these values becomes nil. As soon as keys are deleted, Redis gets unblocked and plugin continues to read data that is already deleted.

[1 [::1]:35468] "get" "/vnf-agent/vpp1589/config/vpp/v2/interfaces/<GRE tunnel #1899>
[1 [::1]:35468] "get" "/vnf-agent/vpp1589/config/vpp/v2/interfaces/<GRE tunnel #1900>
[1 [::1]:35468] "get" "/vnf-agent/vpp1589/config/vpp/v2/interfaces/<GRE tunnel #1901>
[1 [::1]:35468] "get" "/vnf-agent/vpp1589/config/vpp/v2/interfaces/<GRE tunnel #1902>
[1 lua] "del" "/vnf-agent/vpp1589/config/vpp/v2/interfaces/<GRE tunnel #1>" "/vnf-agent/vpp1589/config/vpp/v2/interfaces/<GRE tunnel #2>" <...>
[1 [::1]:35468] "get" "/vnf-agent/vpp1589/config/vpp/v2/interfaces/<GRE tunnel #1903> <-- nil value, SIGSEGV

Expected results:
On nil value watch event CN-Infra protoWatchResp.GetValue() should return error. VPP agent's orchestrator plugin Plugin.watchEvents() should get an error, log it, and continue operating.
Actual results:
proto_watcher_impl.go:GetValue() tries to unmarshall protoWatchResp.BytesWatchResp.value that is nil. VPP-agent crashes with SIGSEGV

[dzvancuks]

Sorry, this was my local changes issue. Did some experiments with orchistrator without checking for nil event value.