Properly deinitialize current input for extension dialogs

If an input and extension is running, while quitting VLC all extensions need to be notified of new NULL input. This will decrease ref count appropriately.

The first choice of doing this would be intf.m:840, where p_current_input is handled:

[[ExtensionsManager getInstance:p_intf] inputChanged:NULL];

But this call cannot be called on the main thread, because of possible deadlocks. But it also should not be called on another thread, as the extension manager might be already gone by then. Now, what?

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