[perl #131383] [IO][MOAR][REGRESSION] .readchars($size) sometimes returns $size+1 chars

[Samantha McVey via RT]

I bisected MoarVM and the offending commit is here:

https://github.com/MoarVM/MoarVM/commit/c98634cf2542874d7daa5b45f77f7de4cf04a081

>From what I see, this commit did not actually cause the root bug, it just 
>exposed it.

The Unicode Database was rebuilt so that NFG_QC=False for Emoji characters, 
which the ♥ is.

Also interestingly:

        <samcv> m:my $f1 = "/tmp/foo121212".IO; $f1.spurt: "fo♥22"; 
$f1.open.readchars(2).say
        <evalable6> samcv, rakudo-moar 0c5fe56cc: OUTPUT: «fo♥22»

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