Fix segfault caused by UnusedLabelCheck #277

Job	Run time
Run all tests (dmd-latest, dmd, ubuntu-22.04, make)	2m 48s
Run all tests (dmd-latest, dmd, ubuntu-22.04, dub, min libdparse)	1m 34s
Run all tests (dmd-latest, dmd, macos-latest, make)	21m 5s
Run all tests (dmd-latest, dmd, macos-latest, dub, min libdparse)	2m 47s
Run all tests (dmd-latest, dmd, windows-latest, make)	4m 16s
Run all tests (dmd-latest, dmd, windows-latest, dub, min libdparse)	3m 6s
Run all tests (ldc-latest, ldmd2, ubuntu-22.04, make)	3m 41s
Run all tests (ldc-latest, ldmd2, ubuntu-22.04, dub, min libdparse)	2m 18s
Run all tests (ldc-latest, ldmd2, macos-latest, make)	4h 25m 15s
Run all tests (ldc-latest, ldmd2, macos-latest, dub, min libdparse)	4m 1s
Run all tests (ldc-latest, ldmd2, windows-latest, make)	8m 4s
Run all tests (ldc-latest, ldmd2, windows-latest, dub, min libdparse)	3m 37s
Run all tests (dmd-latest, gdc-12, ubuntu-22.04, make)	3m 0s
	5h 25m 32s

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