Fix exception when NtTerminateThread is called on the active thread. (#480)

Fixes a bug where `perform_context_switch_work` method erases the thread
from the threads list and call its destructor but doesn't invalidate the
`active_thread` property causing the following switch_to_thread call to
call `active_thread->save(emu);` when `active_thread` is already
destroyed and thus throws an exception when assigning to
`last_registers`.

src/windows-emulator/windows_emulator.cpp

@@ -52,6 +52,7 @@ namespace
     void perform_context_switch_work(windows_emulator& win_emu)
     {
         auto& threads = win_emu.process.threads;
+        auto*& active = win_emu.process.active_thread;
 
         for (auto it = threads.begin(); it != threads.end();)
         {
@@ -61,6 +62,11 @@ namespace
                 continue;
             }
 
+            if (active == &it->second)
+            {
+                active = nullptr;
+            }
+
             const auto [new_it, deleted] = threads.erase(it);
             if (!deleted)
             {