fix issue 19717 - case of segfault due to undetected forward reference

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19717	normal	case of segfault due to undetected forward reference

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#9412"