More gracefully handle killed threads. Still needs some UI love.

src/xenia/debug/debugger.cc

@@ -294,6 +294,9 @@ void Debugger::UpdateThreadExecutionStates(uint32_t override_handle,
   for (auto& it : thread_execution_infos_) {
     auto thread_info = it.second.get();
     auto thread = thread_info->thread;
+    if (!thread) {
+      continue;
+    }
 
     // Grab PPC context.
     // Note that this is only up to date if --store_all_context_values is

src/xenia/debug/ui/debug_window.cc

@@ -995,6 +995,9 @@ void DebugWindow::DrawThreadsPane() {
     auto thread = thread_info->thread;
     bool is_current_thread = thread == state_.thread;
     assert_not_null(thread);  // TODO(benvanik): zombie thread states.
+    if (!thread) {
+      continue;
+    }
     if (is_current_thread && state_.has_changed_thread) {
       ImGui::SetScrollHere();
       state_.has_changed_thread = false;