[Issue 18844] std.utf.decode skips valid character on invalid multibyte sequence

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18844

--- Comment #1 from FeepingCreature <default_357-line at yahoo.de> ---
Repro:

    string s = cast(string) [cast(ubyte) 'ä', 't'];
    size_t i = 0;
    auto ch = decode!(UseReplacementDchar.yes, string)(s, i);
    writefln("ch = %s, i = %s, should be 1", ch, i);

ch = �, i = 2, should be 1.

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