Don't destroy P5 objects after the P5 interpreter was shut down

We run the default_perl5 object's DESTROY method unconditionally in the END.
If after that the GC runs, it may DESTROY wrapped Perl 5 objects. If we
try to decrement their reference count, we're in no-mans-land and may
encounter undesireable effects like segfaults. So don't do that.

lib/Inline/Perl5.pm6

@@ -636,6 +636,7 @@ method init_callbacks {
 }
 
 method sv_refcnt_dec($obj) {
+    return unless $!p5; # Destructor may already have run. Destructors of individual P5 objects no longer work.
     p5_sv_refcnt_dec($!p5, $obj);
 }