[BACKPORT/22.1.x] go/runtime/txpool: Fix crash on early access

[kostko]

Backport of #4638.

[codecov]

Codecov Report

Merging #4639 (8c492c2) into stable/22.1.x (f704c6c) will increase coverage by 0.06%.
The diff coverage is 0.00%.

@@                Coverage Diff                @@
##           stable/22.1.x    #4639      +/-   ##
=================================================
+ Coverage          66.98%   67.05%   +0.06%     
=================================================
  Files                431      431              
  Lines              48805    48818      +13     
=================================================
+ Hits               32694    32733      +39     
+ Misses             12078    12058      -20     
+ Partials            4033     4027       -6     

Impacted Files	Coverage Δ
go/runtime/txpool/txpool.go	74.00% <0.00%> (-3.01%)	⬇️
go/worker/compute/executor/committee/batch.go	64.00% <0.00%> (-8.00%)	⬇️
go/consensus/api/submission.go	64.28% <0.00%> (-4.77%)	⬇️
go/worker/storage/p2p/sync/server.go	74.19% <0.00%> (-3.23%)	⬇️
go/runtime/registry/host.go	66.23% <0.00%> (-2.17%)	⬇️
go/worker/client/committee/node.go	73.54% <0.00%> (-1.30%)	⬇️
go/consensus/tendermint/abci/mux.go	56.33% <0.00%> (-1.10%)	⬇️
go/common/grpc/grpc.go	81.95% <0.00%> (-0.89%)	⬇️
go/runtime/transaction/transaction.go	79.83% <0.00%> (-0.85%)	⬇️
go/consensus/tendermint/roothash/roothash.go	67.44% <0.00%> (-0.64%)	⬇️
... and 19 more

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