go/consensus/tendermint: Fix nil deref in EstimateGas

[kostko]

Fixes #3647.

[codecov]

Codecov Report

Merging #3648 (f23b6cb) into master (b11f535) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3648      +/-   ##
==========================================
- Coverage   67.15%   67.14%   -0.01%     
==========================================
  Files         390      390              
  Lines       36678    36682       +4     
==========================================
  Hits        24631    24631              
- Misses       8574     8576       +2     
- Partials     3473     3475       +2     

Impacted Files	Coverage Δ
go/consensus/api/api.go	14.28% <ø> (ø)
go/consensus/tendermint/abci/mux.go	57.97% <100.00%> (+0.55%)	⬆️
go/consensus/tests/tester.go	98.14% <100.00%> (+0.03%)	⬆️
go/consensus/tendermint/abci/state/state.go	54.54% <0.00%> (-9.10%)	⬇️
go/common/sgx/aesm/aesm.go	62.61% <0.00%> (-6.55%)	⬇️
go/runtime/tagindexer/tagindexer.go	66.27% <0.00%> (-4.66%)	⬇️
go/storage/mkvs/iterator.go	80.29% <0.00%> (-2.92%)	⬇️
go/worker/storage/committee/checkpoint_sync.go	73.92% <0.00%> (-2.73%)	⬇️
go/runtime/transaction/transaction.go	72.36% <0.00%> (-2.64%)	⬇️
go/storage/mkvs/lookup.go	72.04% <0.00%> (-2.16%)	⬇️
... and 21 more

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