[Issue 21815] std.format: invalid UTF-character throws with width but not without

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21815

--- Comment #1 from Berni44 <bugzilla at bernis-buecher.de> ---
Reading the unicode standard, I think, I erred here on what should happen.
Actually, the invalid 0xff character should be replaced by a 0xFFFD-replacement
character in both versions (or both versions should throw, but that is not
recommended any more, because it might permit a denial of service attack).

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