This ensures that the controller GCPtr is non-null by the time we
capture a copy of it for the lambda passed to the request signal's
add_abort_algorithm() method. Currently, the VERIFY() would always fail
when aborting the signal. The alternative to this would be adding a cell
setter to Handle, and ensuring that null handles have a HandleImpl as
well; this seems not worth the hassle right now. Thanks to Lubrsi for
catching this!
Co-authored-by: Luke Wilde <lukew@serenityos.org>