LibCore: Pop the main Core::EventLoop off the stack when destroyed

The main event loop pushes itself onto the event loop stack, and so it
should also pop itself when destroyed.

This will surface attempts to use the event loop stack after the main
event loop has been destroyed.

Userland/Libraries/LibCore/EventLoop.cpp

@@ -293,6 +293,11 @@ EventLoop::EventLoop([[maybe_unused]] MakeInspectable make_inspectable)
 
 EventLoop::~EventLoop()
 {
+    // NOTE: Pop the main event loop off of the stack when destroyed.
+    if (this == s_main_event_loop) {
+        s_event_loop_stack->take_last();
+        s_main_event_loop = nullptr;
+    }
 }
 
 bool connect_to_inspector_server()