cleanup: fix shellcheck warnings

[coryan]

This change is 

[codecov]

Codecov Report

Merging #1467 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@ Coverage Diff @@ ## master #1467 +/- ## ========================================== - Coverage 95.58% 95.55% -0.04%  ========================================== Files 192 195 +3 Lines 15824 15824 ========================================== - Hits 15126 15121 -5  - Misses 698 703 +5 

Impacted Files	Coverage Δ
google/cloud/spanner/internal/log_wrapper.h	75.00% <0.00%> (-9.62%)	⬇️
google/cloud/spanner/internal/spanner_stub.cc	76.34% <0.00%> (-1.08%)	⬇️
google/cloud/spanner/value.h	92.19% <0.00%> (-0.11%)	⬇️
...anner/integration_tests/client_integration_test.cc	97.78% <0.00%> (-0.02%)	⬇️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	91.96% <0.00%> (-0.01%)	⬇️
google/cloud/spanner/transaction.h	100.00% <0.00%> (ø)
google/cloud/spanner/samples/samples.cc	90.21% <0.00%> (ø)
google/cloud/spanner/internal/transaction_impl.h	100.00% <0.00%> (ø)
...oogle/cloud/spanner/internal/database_admin_stub.h	100.00% <0.00%> (ø)
google/cloud/spanner/instance_admin_connection.h	100.00% <0.00%> (ø)
... and 1 more

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[mr-salty]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @devbww, @devjgm, and @scotthart)