PixelPaint: Fix crash on mouse moves

This pr fixes a problem where the application would crash if the mouse
was moved while the editor was still loading the image. With this
change tool related mouse move events are discarded as long as there is
no active tool.

Userland/Applications/PixelPaint/ImageEditor.cpp

@@ -430,6 +430,9 @@ void ImageEditor::mousemove_event(GUI::MouseEvent& event)
         return;
     }
 
+    if (active_tool() == nullptr)
+        return;
+
     auto image_event = event_with_pan_and_scale_applied(event);
     if (on_image_mouse_position_change) {
         on_image_mouse_position_change(image_event.position());