go/worker/common/committee: check if active epoch exists

[ptrus]

Fixes nil ptr defer that can happen if the executor node tries to publish a message before it's synced

[codecov]

Codecov Report

Merging #3432 into master will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #3432   +/-   ##
=======================================
  Coverage   65.98%   65.99%           
=======================================
  Files         371      371           
  Lines       33366    33368    +2     
=======================================
+ Hits        22016    22020    +4     
+ Misses       8110     8095   -15     
- Partials     3240     3253   +13     

Impacted Files	Coverage Δ
go/worker/common/committee/group.go	76.36% <0.00%> (-0.71%)	⬇️
go/oasis-node/cmd/common/metrics/disk.go	65.38% <0.00%> (-19.24%)	⬇️
go/worker/compute/executor/committee/state.go	71.42% <0.00%> (-14.29%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	78.94% <0.00%> (-10.53%)	⬇️
go/consensus/tendermint/abci/state/state.go	54.54% <0.00%> (-9.10%)	⬇️
go/common/grpc/proxy/proxy.go	65.74% <0.00%> (-5.56%)	⬇️
.../consensus/tendermint/apps/epochtime_mock/state.go	70.00% <0.00%> (-5.00%)	⬇️
go/worker/common/committee/runtime_host.go	65.04% <0.00%> (-4.86%)	⬇️
go/worker/storage/service_external.go	48.14% <0.00%> (-3.71%)	⬇️
go/worker/storage/committee/checkpoint_sync.go	75.64% <0.00%> (-2.59%)	⬇️
... and 24 more

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