Fix segfault caused by UnusedLabelCheck #276

Job	Run time
Run all tests (dmd-latest, dmd, ubuntu-22.04, make)	2m 43s
Run all tests (dmd-latest, dmd, ubuntu-22.04, dub, min libdparse)	1m 32s
Run all tests (dmd-latest, dmd, macos-latest, make)	5m 32s
Run all tests (dmd-latest, dmd, macos-latest, dub, min libdparse)	4m 8s
Run all tests (dmd-latest, dmd, windows-latest, make)	3m 56s
Run all tests (dmd-latest, dmd, windows-latest, dub, min libdparse)	2m 52s
Run all tests (ldc-latest, ldmd2, ubuntu-22.04, make)	3m 23s
Run all tests (ldc-latest, ldmd2, ubuntu-22.04, dub, min libdparse)	2m 4s
Run all tests (ldc-latest, ldmd2, macos-latest, make)	7m 4s
Run all tests (ldc-latest, ldmd2, macos-latest, dub, min libdparse)	4m 50s
Run all tests (ldc-latest, ldmd2, windows-latest, make)	8m 9s
Run all tests (ldc-latest, ldmd2, windows-latest, dub, min libdparse)	3m 38s
Run all tests (dmd-latest, gdc-12, ubuntu-22.04, make)	1m 57s
	51m 48s

Provide feedback