unicode2native: Fix conversion to UTF-16 (bug #64139). * liboctave/wrappers/uniconv-wrappers.c (octave_u8_conv_to_encoding_intern): Avoid appending a zero-byte when converting to UTF-* to avoid having to strip a varying number of bytes after the conversion. * scripts/strings/unicode2native.m: Add test for conversion to UTF-16.

%!function out = bug54490 ()
%!  global k;
%!  k = 1;
%!  out = 3;
%!endfunction

%!test <*54490>
%! global k;
%! k = 2;
%! a = [5, 6];
%! a(k) = bug54490 ();
%! assert (a, [5, 3]);
%! k = 2;
%! a = [5, 6];
%! [a(k)] = bug54490 ();
%! assert (a, [5, 3]);
%! clear -global k;  # cleanup after test