macosx: We need the PL_LOCK here (May fix a race we have on startup).

(cherry picked from commit 74a2f792)

modules/gui/macosx/playlist.m

@@ -171,6 +171,7 @@
     NSValue *o_value;
     playlist_t * p_playlist = pl_Yield( VLCIntf );
 
+    PL_LOCK;
     if( item == nil )
     {
         /* root object */
@@ -182,6 +183,7 @@
     }
     if( p_item && index < p_item->i_children && index >= 0 )
         p_return = p_item->pp_children[index];
+    PL_UNLOCK;
 
     vlc_object_release( p_playlist );