boost::asio::signal_set crash for boost 1.89.0

Came across this, possibly "doing it wrong" but possibly a Cobalt bug?

```
#include

#include
#include

boost::cobalt::task signalWait()
{
auto exec = co_await boost::cobalt::this_coro::executor;
boost::asio::signal_set signals(exec, SIGINT);
co_await signals.async_wait(boost::cobalt::use_op);
}

boost::cobalt::main co_main(int argc, char *argv[])
{
co_await boost::cobalt::join(signalWait());
}
```

*Expected behaviour* Typing ^C (SIGINT) will be consumed by the signalWait coroutine and the program will exit normally.

*Observed behaviour* ^C (SIGINT) results in segfault

According to gdb:

```
(gdb) signal 2
Continuing with signal SIGINT.

Program received signal SIGSEGV, Segmentation fault.
0x0000555555568743 in boost::asio::cancellation_signal::emit (this=0x7fffffffbc00, type=boost::asio::cancellation_type::total)
at /opt/boost-1.89.0/include/boost/asio/cancellation_signal.hpp:99
99 handler_->call(type);
(gdb) where
#0 0x0000555555568743 in boost::asio::cancellation_signal::emit (this=0x7fffffffbc00, type=boost::asio::cancellation_type::total)
at /opt/boost-1.89.0/include/boost/asio/cancellation_signal.hpp:99
#1 0x0000555555580990 in work::operator() (this=0x7fffffffb7b0, ec=..., sig=2) at /home/nigel-stewart/dev/cobalt/src/main.cpp:45
#2 0x0000555555581f90 in boost::asio::detail::binder2::operator()(void) (
this=0x7fffffffb7b0) at /opt/boost-1.89.0/include/boost/asio/detail/bind_handler.hpp:181
#3 0x0000555555581edb in boost::asio::detail::handler_work::complete >(boost::asio::detail::binder2 &, work &) (this=0x7fffffffb7e0, function=..., handler=...)
at /opt/boost-1.89.0/include/boost/asio/detail/handler_work.hpp:470
#4 0x0000555555581ae1 in boost::asio::detail::signal_handler::do_complete(void *, boost::asio::detail::operation *, const boost::system::error_code &, std::size_t) (owner=0x5555555c1610, base=0x5555555c1ca0)
at /opt/boost-1.89.0/include/boost/asio/detail/signal_handler.hpp:76
#5 0x0000555555563d46 in boost::asio::detail::scheduler_operation::complete (this=0x5555555c1ca0, owner=0x5555555c1610, ec=..., bytes_transferred=0)
at /opt/boost-1.89.0/include/boost/asio/detail/scheduler_operation.hpp:40
#6 0x0000555555567c4b in boost::asio::detail::scheduler::do_run_one (this=0x5555555c1610, lock=..., this_thread=..., ec=...)
at /opt/boost-1.89.0/include/boost/asio/detail/impl/scheduler.ipp:501
#7 0x00005555555675fa in boost::asio::detail::scheduler::run (this=0x5555555c1610, ec=...) at /opt/boost-1.89.0/include/boost/asio/detail/impl/scheduler.ipp:217
#8 0x00005555555839bf in boost::asio::io_context::run (this=0x7fffffffbaa0) at /opt/boost-1.89.0/include/boost/asio/impl/io_context.ipp:63
#9 0x0000555555580c1a in boost::cobalt::detail::main_promise::run_main (mn=...) at /home/nigel-stewart/dev/cobalt/src/main.cpp:61
#10 0x0000555555569921 in boost::cobalt::detail::main (argc=1, argv=0x7fffffffdcd8) at /home/nigel-stewart/dev/cobalt/include/boost/cobalt/detail/main.hpp:123
```