Program crash: GC destroys an object unexpectedly

[eugene]

Code snippets

```d
class Stopper : StageMachine {

     enum ulong M0_IDLE = 0;
     Signal sg0, sg1;

     this() {
         super("STOPPER");

         Stage init, idle;
         init = addStage("INIT", &stopperInitEnter);
         idle = addStage("IDLE", &stopperIdleEnter);

         init.addReflex("M0", idle);

         idle.addReflex("S0", &stopperIdleS0);
         idle.addReflex("S1", &stopperIdleS1);
     }

     void stopperInitEnter() {
         sg0 = newSignal(Signal.sigInt);
         sg1 = newSignal(Signal.sigTerm);
         msgTo(this, M0_IDLE);
     }
```

The instance of Stopper is created in the scope of main():

```d
void main(string[] args) {

     auto stopper = new Stopper();
     stopper.run();
```

stopperInitEnter(), where sg0 and sg1 are created, is invoked 
inside run() method.

After ~6 seconds from the start (dummy) destructors of sg0 and 
sg1 are called:

    !!! esrc.EventSource.~this() : esrc.Signal (owner STOPPER, fd 
= 24) this @ 0x7fa5410d4f60
    !!! esrc.EventSource.~this() : esrc.Signal (owner STOPPER, fd 
= 25) this @ 0x7fa5410d4f90

Then after pressing ^C (SIGINT) the program gets SIGSEGV, since 
references to sg0 and sg1 are no longer valid (they are "sitting" 
in epoll_event structure).

First I thought I am stupid and I do not see some obvious 
mistake, but...
That crash happens if the program was compiled with dmd 
(v2.097.2).
When using gdc (as well as ldc, both from Debian 8 official 
repo), I do not observe no crashes - program may run for hours 
and after interrupting by ^C it terminates as expected.

And the most strange thing is this - if using gdc with -Os flag, 
the program behaves
exactly as when compiled with fresh dmd - destructors for sg0 and 
sg1 are called soon
after program start.

I do not understand at all why GC considers those sg0 and sg1 as 
unreferenced.
And why old gdc (without -Os) and old ldc do not.