ARROW-14370: [C++] Fix memory leak in SeqMergedGeneratorTestFixture.ErrorItem
ARROW-14370: [C++] Fix memory leak in SeqMergedGeneratorTestFixture.ErrorItem

Closes #11461 from westonpace/bugfix/ARROW-14370--seq-merged-gen-asan-failure

Authored-by: Weston Pace <0dd0706858a011bb30a3fd82b71cee4f9094c502@gmail.com>
Signed-off-by: Krisztián Szűcs <6bab32dc957a05a87930e779f272342e159326fa@gmail.com>

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