Fixed normal bug EXT-4762 ( Crash on exit from viewer in LLPanelPeopleMenus::ContextMenu)

- reason: attempt to delete instance of the LLContextMenu from the LLPanelPeopleMenus::ContextMenu after it was deleted by menu holder on viewer exit.
- fix: check if menu is not dead via stored LLHandle of the menu before deleting.

--HG--
branch : product-engine