[runtime] Fix handling of segfault on sgen thread

Threads without domains that get segfaults will end up in
this handler. It's not safe to call this function with a NULL domain.

See crash below:

```
* thread mono#1, name = 'tid_307', queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=2, address=0x10eff40f8)
  * frame #0: 0x000000010e1510d9 mono-sgen`mono_threads_summarize_execute(ctx=0x0000000000000000, out=0x0000001000000000, hashes=0x0000100000100000, silent=4096, mem="", provided_size=2199023296512) at threads.c:6414
    frame mono#1: 0x000000010e152092 mono-sgen`mono_threads_summarize(ctx=0x000000010effda00, out=0x000000010effdba0, hashes=0x000000010effdb90, silent=0, signal_handler_controller=1, mem=0x0000000000000000, provided_size=0) at threads.c:6508
    frame mono#2: 0x000000010df7c69f mono-sgen`dump_native_stacktrace(signal="SIGSEGV", ctx=0x000000010effef48) at mini-posix.c:1026
    frame mono#3: 0x000000010df7c37f mono-sgen`mono_dump_native_crash_info(signal="SIGSEGV", ctx=0x000000010effef48, info=0x000000010effeee0) at mini-posix.c:1147
    frame mono#4: 0x000000010de720a9 mono-sgen`mono_handle_native_crash(signal="SIGSEGV", ctx=0x000000010effef48, info=0x000000010effeee0) at mini-exceptions.c:3227
    frame mono#5: 0x000000010dd6ac0d mono-sgen`mono_sigsegv_signal_handler_debug(_dummy=11, _info=0x000000010effeee0, context=0x000000010effef48, debug_fault_addr=0xffffffffffffffff) at mini-runtime.c:3574
    frame mono#6: 0x000000010dd6a8d3 mono-sgen`mono_sigsegv_signal_handler(_dummy=11, _info=0x000000010effeee0, context=0x000000010effef48) at mini-runtime.c:3612
    frame mono#7: 0x00007fff73dbdf5a libsystem_platform.dylib`_sigtramp + 26
    frame mono#8: 0x0000000110bb81c1
    frame mono#9: 0x000000011085ffe1
    frame mono#10: 0x000000010dd6d4f3 mono-sgen`mono_jit_runtime_invoke(method=0x00007faae4f01fe8, obj=0x0000000000000000, params=0x00007ffee1eaa180, exc=0x00007ffee1ea9f08, error=0x00007ffee1eaa250) at mini-runtime.c:3215
    frame mono#11: 0x000000010e11509d mono-sgen`do_runtime_invoke(method=0x00007faae4f01fe8, obj=0x0000000000000000, params=0x00007ffee1eaa180, exc=0x0000000000000000, error=0x00007ffee1eaa250) at object.c:2977
    frame mono#12: 0x000000010e10d961 mono-sgen`mono_runtime_invoke_checked(method=0x00007faae4f01fe8, obj=0x0000000000000000, params=0x00007ffee1eaa180, error=0x00007ffee1eaa250) at object.c:3145
    frame mono#13: 0x000000010e11aa58 mono-sgen`do_exec_main_checked(method=0x00007faae4f01fe8, args=0x000000010f0003e8, error=0x00007ffee1eaa250) at object.c:5042
    frame mono#14: 0x000000010e118803 mono-sgen`mono_runtime_exec_main_checked(method=0x00007faae4f01fe8, args=0x000000010f0003e8, error=0x00007ffee1eaa250) at object.c:5138
    frame mono#15: 0x000000010e118856 mono-sgen`mono_runtime_run_main_checked(method=0x00007faae4f01fe8, argc=2, argv=0x00007ffee1eaa760, error=0x00007ffee1eaa250) at object.c:4599
    frame mono#16: 0x000000010de1db2f mono-sgen`mono_jit_exec_internal(domain=0x00007faae4f00860, assembly=0x00007faae4c02ab0, argc=2, argv=0x00007ffee1eaa760) at driver.c:1298
    frame mono#17: 0x000000010de1d95d mono-sgen`mono_jit_exec(domain=0x00007faae4f00860, assembly=0x00007faae4c02ab0, argc=2, argv=0x00007ffee1eaa760) at driver.c:1257
    frame mono#18: 0x000000010de2257f mono-sgen`main_thread_handler(user_data=0x00007ffee1eaa6a0) at driver.c:1375
    frame mono#19: 0x000000010de20852 mono-sgen`mono_main(argc=3, argv=0x00007ffee1eaa758) at driver.c:2551
    frame mono#20: 0x000000010dd56d7e mono-sgen`mono_main_with_options(argc=3, argv=0x00007ffee1eaa758) at main.c:50
    frame mono#21: 0x000000010dd5638d mono-sgen`main(argc=3, argv=0x00007ffee1eaa758) at main.c:406
    frame mono#22: 0x00007fff73aaf015 libdyld.dylib`start + 1
    frame mono#23: 0x00007fff73aaf015 libdyld.dylib`start + 1
  thread mono#2, name = 'SGen worker'
    frame #0: 0x000000010e2afd77 mono-sgen`mono_get_hazardous_pointer(pp=0x0000000000000178, hp=0x000000010ef87618, hazard_index=0) at hazard-pointer.c:208
    frame mono#1: 0x000000010e0b28e1 mono-sgen`mono_jit_info_table_find_internal(domain=0x0000000000000000, addr=0x00007fff73bffa16, try_aot=1, allow_trampolines=1) at jit-info.c:304
    frame mono#2: 0x000000010dd6aa5f mono-sgen`mono_sigsegv_signal_handler_debug(_dummy=11, _info=0x000070000fb81c58, context=0x000070000fb81cc0, debug_fault_addr=0x000000010e28fb20) at mini-runtime.c:3540
    frame mono#3: 0x000000010dd6a8d3 mono-sgen`mono_sigsegv_signal_handler(_dummy=11, _info=0x000070000fb81c58, context=0x000070000fb81cc0) at mini-runtime.c:3612
    frame mono#4: 0x00007fff73dbdf5a libsystem_platform.dylib`_sigtramp + 26
    frame mono#5: 0x00007fff73bffa17 libsystem_kernel.dylib`__psynch_cvwait + 11
    frame mono#6: 0x00007fff73dc8589 libsystem_pthread.dylib`_pthread_cond_wait + 732
    frame mono#7: 0x000000010e28d76d mono-sgen`mono_os_cond_wait(cond=0x000000010e44c9d8, mutex=0x000000010e44c998) at mono-os-mutex.h:168
    frame mono#8: 0x000000010e28df4f mono-sgen`get_work(worker_index=0, work_context=0x000070000fb81ee0, do_idle=0x000070000fb81ed4, job=0x000070000fb81ec8) at sgen-thread-pool.c:165
    frame mono#9: 0x000000010e28d2cb mono-sgen`thread_func(data=0x0000000000000000) at sgen-thread-pool.c:196
    frame mono#10: 0x00007fff73dc7661 libsystem_pthread.dylib`_pthread_body + 340
    frame mono#11: 0x00007fff73dc750d libsystem_pthread.dylib`_pthread_start + 377
    frame mono#12: 0x00007fff73dc6bf9 libsystem_pthread.dylib`thread_start + 13
  thread mono#3, name = 'Finalizer'
    frame #0: 0x00007fff73bf6246 libsystem_kernel.dylib`semaphore_wait_trap + 10
    frame mono#1: 0x000000010e1d9c0a mono-sgen`mono_os_sem_wait(sem=0x000000010e43e400, flags=MONO_SEM_FLAGS_ALERTABLE) at mono-os-semaphore.h:84
    frame mono#2: 0x000000010e1d832d mono-sgen`mono_coop_sem_wait(sem=0x000000010e43e400, flags=MONO_SEM_FLAGS_ALERTABLE) at mono-coop-semaphore.h:41
    frame mono#3: 0x000000010e1da787 mono-sgen`finalizer_thread(unused=0x0000000000000000) at gc.c:920
    frame mono#4: 0x000000010e152919 mono-sgen`start_wrapper_internal(start_info=0x0000000000000000, stack_ptr=0x000070000fd85000) at threads.c:1178
    frame mono#5: 0x000000010e1525b6 mono-sgen`start_wrapper(data=0x00007faae4f31bd0) at threads.c:1238
    frame mono#6: 0x00007fff73dc7661 libsystem_pthread.dylib`_pthread_body + 340
    frame mono#7: 0x00007fff73dc750d libsystem_pthread.dylib`_pthread_start + 377
    frame mono#8: 0x00007fff73dc6bf9 libsystem_pthread.dylib`thread_start + 13
  thread mono#4
    frame #0: 0x00007fff73c0028a libsystem_kernel.dylib`__workq_kernreturn + 10
    frame mono#1: 0x00007fff73dc7009 libsystem_pthread.dylib`_pthread_wqthread + 1035
    frame mono#2: 0x00007fff73dc6be9 libsystem_pthread.dylib`start_wqthread + 13
(lldb)
```

mono/mini/mini-runtime.c

@@ -3496,7 +3496,8 @@ MONO_SIG_HANDLER_FUNC_DEBUG (static, mono_sigsegv_signal_handler_debug)
 MONO_SIG_HANDLER_FUNC (, mono_sigsegv_signal_handler)
 #endif
 {
-	MonoJitInfo *ji;
+	MonoJitInfo *ji = NULL;
+	MonoDomain *domain = mono_domain_get ();
 	MonoJitTlsData *jit_tls = mono_tls_get_jit_tls ();
 	gpointer fault_addr = NULL;
 #ifdef HAVE_SIG_INFO
@@ -3537,7 +3538,8 @@ MONO_SIG_HANDLER_FUNC (, mono_sigsegv_signal_handler)
 	}
 #endif
 
-	ji = mono_jit_info_table_find_internal (mono_domain_get (), mono_arch_ip_from_context (ctx), TRUE, TRUE);
+	if (domain)
+		ji = mono_jit_info_table_find_internal (domain, mono_arch_ip_from_context (ctx), TRUE, TRUE);
 
 #ifdef MONO_ARCH_SIGSEGV_ON_ALTSTACK
 	if (mono_handle_soft_stack_ovf (jit_tls, ji, ctx, info, (guint8*)info->si_addr))