Don't complain about utf8 multibyte characters split by chunk boundaries

Fixes: https://rt.perl.org/rt3/Public/Bug/Display.html?id=117841

Test with an installed nqp+rakudo is:

perl6 -e 'use Test; for 1..12 -> $x { ok( qqx[perl6 -e "say 1 x $_,q|—|" | cat] ~~ /^1+\—\s*$/, "Test for $_ bytes + utf8 char") for map { 2**$x - 1 }, ^5 }'