[debugger-agent] Ignore thread_end event if thread is detached

[debugger-agent] Ignore thread_end event if thread is detached

The thread_stopped profiler event can be raised by the thread_info_key_dtor tls
key destructor when the thread is already doesn't have a domain set.  In that
case, don't call process_profiler_event since it cannot handle a thread with
null TLS values.

Addresses https://github.com/xamarin/xamarin-android/issues/2920
with the following stack trace

```
* thread #20, name = 'Filter', stop reason = signal SIGSEGV: invalid address (fault address: 0xbc)
  * frame #0: libmonosgen-2.0.so`mono_class_vtable_checked(domain=0x0000000000000000, klass=0x0000007200230648, error=0x00000071e92f9178) at object.c:1890
    frame #1: libmonosgen-2.0.so`get_current_thread_ptr_for_domain(domain=0x0000000000000000, thread=0x00000071ebfec508) at threads.c:595
    frame #2: libmonosgen-2.0.so`mono_thread_current at threads.c:1939
    frame #3: libmonosgen-2.0.so`process_event(event=<unavailable>, arg=<unavailable>, il_offset=<unavailable>, ctx=<unavailable>, events=<unavailable>, suspend_policy=<unavailable>) at debugger-agent.c:3715
    frame #4: libmonosgen-2.0.so`thread_end [inlined] process_profiler_event(event=EVENT_KIND_THREAD_DEATH, arg=0x00000071ebfec508) at debugger-agent.c:3875
    frame #5: libmonosgen-2.0.so`thread_end(prof=<unavailable>, tid=<unavailable>) at debugger-agent.c:3991
    frame #6: libmonosgen-2.0.so`mono_profiler_raise_thread_stopped(tid=<unavailable>) at profiler-events.h:105
    frame #7: libmonosgen-2.0.so`mono_thread_detach_internal(thread=<unavailable>) at threads.c:979
    frame #8: libmonosgen-2.0.so`thread_detach(info=0x00000071e949a000) at threads.c:3215
    frame #9: libmonosgen-2.0.so`unregister_thread(arg=<unavailable>) at mono-threads.c:544
    frame #10: libmonosgen-2.0.so`thread_info_key_dtor(arg=0x00000071e949a000) at mono-threads.c:774
    frame #11: 0x00000072899c58e8 libc.so`pthread_key_clean_all() + 124
    frame #12: 0x00000072899c5374 libc.so`pthread_exit + 76
    frame #13: 0x00000072899c5264 libc.so`__pthread_start(void*) + 44
    frame #14: 0x000000728996617c libc.so`__start_thread + 72
```