Fix segfault caused by UnusedLabelCheck #275

Job	Run time
Run all tests (dmd-latest, dmd, ubuntu-22.04, make)	2m 37s
Run all tests (dmd-latest, dmd, ubuntu-22.04, dub, min libdparse)	1m 39s
Run all tests (dmd-latest, dmd, macos-latest, make)	7m 55s
Run all tests (dmd-latest, dmd, macos-latest, dub, min libdparse)	2m 53s
Run all tests (dmd-latest, dmd, windows-latest, make)	3m 58s
Run all tests (dmd-latest, dmd, windows-latest, dub, min libdparse)	2m 56s
Run all tests (ldc-latest, ldmd2, ubuntu-22.04, make)	3m 16s
Run all tests (ldc-latest, ldmd2, ubuntu-22.04, dub, min libdparse)	1m 57s
Run all tests (ldc-latest, ldmd2, macos-latest, make)	8m 55s
Run all tests (ldc-latest, ldmd2, macos-latest, dub, min libdparse)	3m 13s
Run all tests (ldc-latest, ldmd2, windows-latest, make)	7m 52s
Run all tests (ldc-latest, ldmd2, windows-latest, dub, min libdparse)	3m 55s
Run all tests (dmd-latest, gdc-12, ubuntu-22.04, make)	1m 59s
	53m 5s

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