Fix "Object in QAST::WVal, but not in SC" for function pointers

While adding the sub to a newly created SC is harmful in the normal case,
it's vital when dealing with function pointers return from native code as
those subs will not be added to an SC by the compiler.

lib/NativeCall.pm6

@@ -464,6 +464,13 @@ my role Native[Routine $r, $libname where Str|Callable|List|IO::Path|Distributio
 
     method !create-optimized-call() {
         $setup-lock.protect: {
+            unless nqp::defined(nqp::getobjsc(self)) {
+                my $sc := nqp::createsc('NativeCallSub' ~ nqp::objectid(self));
+                nqp::setobjsc(self, $sc);
+                my int $idx = nqp::scobjcount($sc);
+                nqp::scsetobj($sc, $idx, self);
+            }
+
             my $block := $!setup == 2
                 ?? self!create-jit-compiled-function-body($r)
                 !! self!create-function-body($r);