fix memleak decimal #6288

Job	Run time
test-new-cases / validate_tests	1m 20s
test-new-cases / run-tests (macos)	6m 13s
test-new-cases / run-tests (windows)	16m 25s
test-new-cases / run-tests (linux)	1h 11m 40s
	1h 35m 38s

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