Fix passing empty strings and bufs to Perl 5.

A 0 len parameter passed to newSVpv makes perl5 use strlen() to
determine len. We don't need this magic anymore, since we determine the
length of the buffer in Perl 6 code already for binary safety.

Works around an apparent bug in Rakudo. The pointer to the supposedly
empty string points to random garbage now.

p5helper.c

@@ -144,14 +144,11 @@ SV *p5_float_to_sv(PerlInterpreter *my_perl, double value) {
 }
 
 SV *p5_str_to_sv(PerlInterpreter *my_perl, STRLEN len, char* value) {
-    SV * const sv = newSVpv(value, len);
-    SvUTF8_on(sv);
-    return sv;
+    return newSVpvn_flags(value, len, SVf_UTF8);
 }
 
 SV *p5_buf_to_sv(PerlInterpreter *my_perl, STRLEN len, char* value) {
-    SV * const sv = newSVpv(value, len);
-    return sv;
+    return newSVpvn_flags(value, len, 0);
 }
 
 I32 p5_av_top_index(PerlInterpreter *my_perl, AV *av) {

t/p6_to_p5.t

@@ -4,7 +4,7 @@ use v6;
 use Test;
 use Inline::Perl5;
 
-plan 16;
+plan 17;
 
 my $p5 = Inline::Perl5.new();
 $p5.run(q:heredoc/PERL5/);
@@ -16,7 +16,17 @@ $p5.run(q:heredoc/PERL5/);
 class Foo {
 }
 
-for ('abcö', Buf.new('äbc'.encode('latin-1')), 24, 2.4.Num, [1, 2], { a => 1, b => 2}, Any, Foo.new) -> $obj {
+for (
+    '',
+    'abcö',
+    Buf.new('äbc'.encode('latin-1')),
+    24,
+    2.4.Num,
+    [1, 2],
+    { a => 1, b => 2},
+    Any,
+    Foo.new,
+) -> $obj {
     is_deeply $p5.call('identity', $obj), $obj, "Can round-trip " ~ $obj.^name;
 }