Fix segfault caused by UnusedLabelCheck #274

Job	Run time
Run all tests (dmd-latest, dmd, ubuntu-22.04, make)	2m 47s
Run all tests (dmd-latest, dmd, ubuntu-22.04, dub, min libdparse)	1m 35s
Run all tests (dmd-latest, dmd, macos-latest, make)	4m 6s
Run all tests (dmd-latest, dmd, macos-latest, dub, min libdparse)	2m 59s
Run all tests (dmd-latest, dmd, windows-latest, make)	4m 10s
Run all tests (dmd-latest, dmd, windows-latest, dub, min libdparse)	3m 25s
Run all tests (ldc-latest, ldmd2, ubuntu-22.04, make)	3m 13s
Run all tests (ldc-latest, ldmd2, ubuntu-22.04, dub, min libdparse)	1m 58s
Run all tests (ldc-latest, ldmd2, macos-latest, make)	12m 1s
Run all tests (ldc-latest, ldmd2, macos-latest, dub, min libdparse)	3m 45s
Run all tests (ldc-latest, ldmd2, windows-latest, make)	7m 44s
Run all tests (ldc-latest, ldmd2, windows-latest, dub, min libdparse)	3m 32s
Run all tests (dmd-latest, gdc-12, ubuntu-22.04, make)	2m 55s
	54m 10s

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