macosx: Fix a deadlock in the broken -[VLCPlaylist isItem: inNode: checkItemExistence:]. Fixes #1711 (Deleting a playing item's parent item result in a trivial deadlock on Mac OS X.)

Kudoz to sam and Meuuh

Note, this commit may create some assert in previously working code. That does mean that this code wasn't properly working, and that it lacks a PL_LOCK. I prefer nice assert() over races.

Should fix #1708 (Adding a folder in the Mac OS X VLC *media library* result in an assertion failure). Thanks to upcoming commit "playlist: Use PL_ASSERT_LOCKED where the playlist lock should be held.".

playlist: Hold the lock at creation time to conform to playlist_NodesPairCreate requirement to have the pl lock held.

playlist: Use PL_LOCK_IF and PL_UNLOCK_IF to perform some more checks on lock state. And fix a unlocked usage of get_current_status_item().

That may fix or help to fix some input item leaks.

Test for the right thing before tring to do anything.
Check malloc return value (fix potential segfault).

Fix #1704

playlist: Don't export playlist_ItemNewFromInput as it is not used, and don't use pl_Yield from here.

Regression if I recall well.

This might requires some other adjustements.