Add a weaken() cycle forgotten in d63c9e6

The reason this has not been a problem is because thread spawn in DESTROY is
practically unheard of. Nevertheless if one throws the following in, the
problem becomes apparent: ( not committing this test, because... just no. )

--- a/t/storage/txn_scope_guard.t
+++ b/t/storage/txn_scope_guard.t
@@ -247,5 +247,7 @@ require DBICTest::AntiPattern::NullObject;
   my @arg_capture;
   {
+    use threads;
     local $SIG{__WARN__} = sub {
+      threads->new(sub { sleep 1})->join;
       package DB;

lib/DBIx/Class/_Util.pm

@@ -366,6 +366,9 @@ sub is_exception ($) {
       values %$destruction_registry
     };
 
+    weaken( $destruction_registry->{$_} )
+      for keys %$destruction_registry;
+
     # Dummy NEXTSTATE ensuring the all temporaries on the stack are garbage
     # collected before leaving this scope. Depending on the code above, this
     # may very well be just a preventive measure guarding future modifications