Fix segfault caused by UnusedLabelCheck #279

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 47s
Run all tests (dmd-latest, dmd, macos-latest, make)	8m 50s
Run all tests (dmd-latest, dmd, macos-latest, dub, min libdparse)	3m 53s
Run all tests (dmd-latest, dmd, windows-latest, make)	4m 16s
Run all tests (dmd-latest, dmd, windows-latest, dub, min libdparse)	3m 2s
Run all tests (ldc-latest, ldmd2, ubuntu-22.04, make)	3m 37s
Run all tests (ldc-latest, ldmd2, ubuntu-22.04, dub, min libdparse)	2m 1s
Run all tests (ldc-latest, ldmd2, macos-latest, make)	5m 38s
Run all tests (ldc-latest, ldmd2, macos-latest, dub, min libdparse)	3m 9s
Run all tests (ldc-latest, ldmd2, windows-latest, make)	7m 44s
Run all tests (ldc-latest, ldmd2, windows-latest, dub, min libdparse)	3m 39s
Run all tests (dmd-latest, gdc-12, ubuntu-22.04, make)	2m 34s
	52m 57s

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