go/runtime/client: Fix possible panic on shutdown

[kostko]

No description provided.

[codecov]

Codecov Report

Merging #3271 into master will increase coverage by 0.11%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master    #3271      +/-   ##
==========================================
+ Coverage   67.73%   67.85%   +0.11%     
==========================================
  Files         371      371              
  Lines       36884    36889       +5     
==========================================
+ Hits        24984    25031      +47     
+ Misses       8712     8670      -42     
  Partials     3188     3188              

Impacted Files	Coverage Δ
go/runtime/client/client.go	69.19% <30.76%> (-1.80%)	⬇️
go/runtime/client/watcher.go	70.54% <50.00%> (-0.23%)	⬇️
go/storage/mkvs/checkpoint/restorer.go	82.45% <0.00%> (-10.53%)	⬇️
go/oasis-node/cmd/common/metrics/resource.go	84.00% <0.00%> (-8.00%)	⬇️
go/consensus/tendermint/abci/state/state.go	61.53% <0.00%> (-7.70%)	⬇️
go/common/grpc/policy/policy.go	64.38% <0.00%> (-6.85%)	⬇️
go/worker/storage/committee/checkpoint_sync.go	74.55% <0.00%> (-6.10%)	⬇️
go/storage/mkvs/syncer/proof.go	83.92% <0.00%> (-3.58%)	⬇️
go/storage/client/client.go	71.68% <0.00%> (-2.22%)	⬇️
go/worker/compute/executor/committee/node.go	64.83% <0.00%> (-1.38%)	⬇️
... and 32 more

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