Fix segfault caused by UnusedLabelCheck #278

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 29s
Run all tests (dmd-latest, dmd, macos-latest, make)	3m 43s
Run all tests (dmd-latest, dmd, macos-latest, dub, min libdparse)	3m 29s
Run all tests (dmd-latest, dmd, windows-latest, make)	3m 42s
Run all tests (dmd-latest, dmd, windows-latest, dub, min libdparse)	2m 52s
Run all tests (ldc-latest, ldmd2, ubuntu-22.04, make)	3m 22s
Run all tests (ldc-latest, ldmd2, ubuntu-22.04, dub, min libdparse)	1m 56s
Run all tests (ldc-latest, ldmd2, macos-latest, make)	4m 40s
Run all tests (ldc-latest, ldmd2, macos-latest, dub, min libdparse)	3m 47s
Run all tests (ldc-latest, ldmd2, windows-latest, make)	7m 54s
Run all tests (ldc-latest, ldmd2, windows-latest, dub, min libdparse)	3m 40s
Run all tests (dmd-latest, gdc-12, ubuntu-22.04, make)	3m 0s
	46m 21s

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