macosx: Improve management of VLCInputItem's internal input_item_t

There are a couple of issues with the internal handling of VLCInputItem's internal input_item_t:

• We silently fail when the internal point is NULL when it shouldn't be, rather than asserting that it shouldn't be NULL
• We run into potential double-free conditions in the dealloc call to VLCInputItem
• We don't assign the returned pointer from input_item_Hold in the VLCInputItem init

Signed-off-by: Claudio Cambra developer@claudiocambra.com