fix issue 18794 - Compiling with -O causes runtime segfault

[aG0aep6G]

Just adding a test. The issue has apparently been fixed by d80e14b (PR #9656).

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	18794	major	Compiling with -O causes runtime segfault

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#9658"