Fix the lib/encode.t subtest 6 failure as reported by Andreas
Koenig, gmagical substr() wasn't propagating UTF8ness. The bug
was unearthed by change 7182, as was a bug in HTML::Entities.
p4raw-id: //depot/perl@7202
if (rem + offs > len)
rem = len - offs;
sv_setpvn(sv, tmps + offs, (STRLEN)rem);
+ if (DO_UTF8(lsv))
+ SvUTF8_on(sv);
return 0;
}