Fix issue 17874 - Static arrays & memset

[LemonBoy]

Hopefully completes what #3784 started.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	17874	major	Segmentation fault when constructing a struct with a static array of structs containing floats

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#8092"