fix issue 22867 - std.utf.decode changes offset despite error.

[aG0aep6G]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	22867	normal	std.utf.decode changes offset despite error.

Testing this PR locally

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

dub run digger -- build "master + phobos#8408"