[perl #131379] [IO] read from file containing 1-char of malformed UTF returns empty string

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# New Ticket Created by  Zoffix Znet 
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This seem to have existed for a while and isn't due to encoding refactor.

01:24   c: 2017.04 with '/tmp/foo999'.IO { .spurt: Buf.new: 200; dd .open.slurp 
}
01:24   committable6    Zoffix, ¦2017.04: «""»
01:24   Zoffix  m: with '/tmp/foo999'.IO { .spurt: Buf.new: 200; dd .open.slurp 
}
01:24   camelia rakudo-moar 094e77: OUTPUT: «""␤»


Note that for file with two of such chars, the behaviour is what I'd expect it 
to be: throwing malformed
UTF8 error, same as you get when decoding Buf.new(200):


01:27           m: with '/tmp/foo999'.IO { .spurt: Buf.new: 200, 200; dd 
.open.slurp }
01:27   camelia rakudo-moar 094e77: OUTPUT: «Malformed UTF-8␤  in block <unit> 
at <tmp> line 1␤␤»
01:25           m: Buf.new(200).decode
01:25   camelia rakudo-moar 094e77: OUTPUT: «Malformed termination of UTF-8 
string␤  in block <unit> at <tmp> line 1␤␤»