Crash when callback is disconnected in a dll

When using signals2 across multiple DDLs, everything works fine until the dlls gets unloaded, which results in crash the next time we dispatch the signal.
I found this [stackoverflow post](https://stackoverflow.com/questions/28703432/access-violation-when-using-boostsignals2-and-unloading-dlls) which describes the same issue i'm having.

The test sample:

Shared.cpp:
```CPP
#include
#include
#include

struct SomeInterface
{
boost::signals2::signal signal;
};

boost::signals2::connection con;

void init(SomeInterface* iface)
{
con = iface->signal.connect([] { std::cout << "Hello, Shared!" << std::endl; });
}

void shutdown()
{
con.disconnect();
}

BOOST_DLL_AUTO_ALIAS(init);
BOOST_DLL_AUTO_ALIAS(shutdown);
```

Host.cpp:
```CPP
#include
#include
#include

struct SomeInterface
{
boost::signals2::signal signal;
};

int main()
{
SomeInterface* iface = new SomeInterface;

{
boost::dll::shared_library dll("Shared", boost::dll::load_mode::append_decorations);

auto init = dll.get("init");
auto shutdown = dll.get("shutdown");

init(iface);
iface->signal();
shutdown();
}

iface->signal();

return 0;
}
````